* [PATCH v3 1/4] mm: Move FOLL_* defs to mm_types.h
2022-12-02 9:43 [PATCH v3 0/4] iov_iter: Add extraction helpers David Howells
@ 2022-12-02 9:43 ` David Howells
2022-12-02 9:43 ` [PATCH v3 2/4] iov_iter: Add a function to extract a page list from an iterator David Howells
1 sibling, 0 replies; 6+ messages in thread
From: David Howells @ 2022-12-02 9:43 UTC (permalink / raw)
To: Al Viro
Cc: Matthew Wilcox, John Hubbard, Christoph Hellwig, linux-mm,
linux-fsdevel, dhowells, Christoph Hellwig, Matthew Wilcox,
Jeff Layton, Logan Gunthorpe, linux-fsdevel, linux-kernel
Move FOLL_* definitions to linux/mm_types.h to make them more accessible
without having to drag in all of linux/mm.h and everything that drags in
too[1].
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: linux-mm@kvack.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/linux-fsdevel/Y1%2FhSO+7kAJhGShG@casper.infradead.org/ [1]
Link: https://lore.kernel.org/r/166732025009.3186319.3402781784409891214.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166869688542.3723671.10243929000823258622.stgit@warthog.procyon.org.uk/ # rfc
---
include/linux/mm.h | 74 ----------------------------------------------
include/linux/mm_types.h | 73 +++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 73 insertions(+), 74 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8bbcccbc5565..7a7a287818ad 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2941,80 +2941,6 @@ static inline vm_fault_t vmf_error(int err)
struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
unsigned int foll_flags);
-#define FOLL_WRITE 0x01 /* check pte is writable */
-#define FOLL_TOUCH 0x02 /* mark page accessed */
-#define FOLL_GET 0x04 /* do get_page on page */
-#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
-#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
-#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO
- * and return without waiting upon it */
-#define FOLL_NOFAULT 0x80 /* do not fault in pages */
-#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
-#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
-#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
-#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
-#define FOLL_ANON 0x8000 /* don't do file mappings */
-#define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */
-#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */
-#define FOLL_PIN 0x40000 /* pages must be released via unpin_user_page */
-#define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */
-
-/*
- * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
- * other. Here is what they mean, and how to use them:
- *
- * FOLL_LONGTERM indicates that the page will be held for an indefinite time
- * period _often_ under userspace control. This is in contrast to
- * iov_iter_get_pages(), whose usages are transient.
- *
- * FIXME: For pages which are part of a filesystem, mappings are subject to the
- * lifetime enforced by the filesystem and we need guarantees that longterm
- * users like RDMA and V4L2 only establish mappings which coordinate usage with
- * the filesystem. Ideas for this coordination include revoking the longterm
- * pin, delaying writeback, bounce buffer page writeback, etc. As FS DAX was
- * added after the problem with filesystems was found FS DAX VMAs are
- * specifically failed. Filesystem pages are still subject to bugs and use of
- * FOLL_LONGTERM should be avoided on those pages.
- *
- * FIXME: Also NOTE that FOLL_LONGTERM is not supported in every GUP call.
- * Currently only get_user_pages() and get_user_pages_fast() support this flag
- * and calls to get_user_pages_[un]locked are specifically not allowed. This
- * is due to an incompatibility with the FS DAX check and
- * FAULT_FLAG_ALLOW_RETRY.
- *
- * In the CMA case: long term pins in a CMA region would unnecessarily fragment
- * that region. And so, CMA attempts to migrate the page before pinning, when
- * FOLL_LONGTERM is specified.
- *
- * FOLL_PIN indicates that a special kind of tracking (not just page->_refcount,
- * but an additional pin counting system) will be invoked. This is intended for
- * anything that gets a page reference and then touches page data (for example,
- * Direct IO). This lets the filesystem know that some non-file-system entity is
- * potentially changing the pages' data. In contrast to FOLL_GET (whose pages
- * are released via put_page()), FOLL_PIN pages must be released, ultimately, by
- * a call to unpin_user_page().
- *
- * FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different
- * and separate refcounting mechanisms, however, and that means that each has
- * its own acquire and release mechanisms:
- *
- * FOLL_GET: get_user_pages*() to acquire, and put_page() to release.
- *
- * FOLL_PIN: pin_user_pages*() to acquire, and unpin_user_pages to release.
- *
- * FOLL_PIN and FOLL_GET are mutually exclusive for a given function call.
- * (The underlying pages may experience both FOLL_GET-based and FOLL_PIN-based
- * calls applied to them, and that's perfectly OK. This is a constraint on the
- * callers, not on the pages.)
- *
- * FOLL_PIN should be set internally by the pin_user_pages*() APIs, never
- * directly by the caller. That's in order to help avoid mismatches when
- * releasing pages: get_user_pages*() pages must be released via put_page(),
- * while pin_user_pages*() pages must be released via unpin_user_page().
- *
- * Please see Documentation/core-api/pin_user_pages.rst for more information.
- */
-
static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
{
if (vm_fault & VM_FAULT_OOM)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 500e536796ca..0c80a5ad6e6a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1003,4 +1003,77 @@ enum fault_flag {
typedef unsigned int __bitwise zap_flags_t;
+/*
+ * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each
+ * other. Here is what they mean, and how to use them:
+ *
+ * FOLL_LONGTERM indicates that the page will be held for an indefinite time
+ * period _often_ under userspace control. This is in contrast to
+ * iov_iter_get_pages(), whose usages are transient.
+ *
+ * FIXME: For pages which are part of a filesystem, mappings are subject to the
+ * lifetime enforced by the filesystem and we need guarantees that longterm
+ * users like RDMA and V4L2 only establish mappings which coordinate usage with
+ * the filesystem. Ideas for this coordination include revoking the longterm
+ * pin, delaying writeback, bounce buffer page writeback, etc. As FS DAX was
+ * added after the problem with filesystems was found FS DAX VMAs are
+ * specifically failed. Filesystem pages are still subject to bugs and use of
+ * FOLL_LONGTERM should be avoided on those pages.
+ *
+ * FIXME: Also NOTE that FOLL_LONGTERM is not supported in every GUP call.
+ * Currently only get_user_pages() and get_user_pages_fast() support this flag
+ * and calls to get_user_pages_[un]locked are specifically not allowed. This
+ * is due to an incompatibility with the FS DAX check and
+ * FAULT_FLAG_ALLOW_RETRY.
+ *
+ * In the CMA case: long term pins in a CMA region would unnecessarily fragment
+ * that region. And so, CMA attempts to migrate the page before pinning, when
+ * FOLL_LONGTERM is specified.
+ *
+ * FOLL_PIN indicates that a special kind of tracking (not just page->_refcount,
+ * but an additional pin counting system) will be invoked. This is intended for
+ * anything that gets a page reference and then touches page data (for example,
+ * Direct IO). This lets the filesystem know that some non-file-system entity is
+ * potentially changing the pages' data. In contrast to FOLL_GET (whose pages
+ * are released via put_page()), FOLL_PIN pages must be released, ultimately, by
+ * a call to unpin_user_page().
+ *
+ * FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different
+ * and separate refcounting mechanisms, however, and that means that each has
+ * its own acquire and release mechanisms:
+ *
+ * FOLL_GET: get_user_pages*() to acquire, and put_page() to release.
+ *
+ * FOLL_PIN: pin_user_pages*() to acquire, and unpin_user_pages to release.
+ *
+ * FOLL_PIN and FOLL_GET are mutually exclusive for a given function call.
+ * (The underlying pages may experience both FOLL_GET-based and FOLL_PIN-based
+ * calls applied to them, and that's perfectly OK. This is a constraint on the
+ * callers, not on the pages.)
+ *
+ * FOLL_PIN should be set internally by the pin_user_pages*() APIs, never
+ * directly by the caller. That's in order to help avoid mismatches when
+ * releasing pages: get_user_pages*() pages must be released via put_page(),
+ * while pin_user_pages*() pages must be released via unpin_user_page().
+ *
+ * Please see Documentation/core-api/pin_user_pages.rst for more information.
+ */
+#define FOLL_WRITE 0x01 /* check pte is writable */
+#define FOLL_TOUCH 0x02 /* mark page accessed */
+#define FOLL_GET 0x04 /* do get_page on page */
+#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
+#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
+#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO
+ * and return without waiting upon it */
+#define FOLL_NOFAULT 0x80 /* do not fault in pages */
+#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
+#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
+#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
+#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
+#define FOLL_ANON 0x8000 /* don't do file mappings */
+#define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */
+#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */
+#define FOLL_PIN 0x40000 /* pages must be released via unpin_user_page */
+#define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */
+
#endif /* _LINUX_MM_TYPES_H */
^ permalink raw reply [flat|nested] 6+ messages in thread* [PATCH v3 2/4] iov_iter: Add a function to extract a page list from an iterator
2022-12-02 9:43 [PATCH v3 0/4] iov_iter: Add extraction helpers David Howells
2022-12-02 9:43 ` [PATCH v3 1/4] mm: Move FOLL_* defs to mm_types.h David Howells
@ 2022-12-02 9:43 ` David Howells
2022-12-06 16:43 ` Logan Gunthorpe
2022-12-06 18:35 ` David Howells
1 sibling, 2 replies; 6+ messages in thread
From: David Howells @ 2022-12-02 9:43 UTC (permalink / raw)
To: Al Viro
Cc: Christoph Hellwig, John Hubbard, Matthew Wilcox, linux-fsdevel,
linux-mm, dhowells, Christoph Hellwig, Matthew Wilcox,
Jeff Layton, Logan Gunthorpe, linux-fsdevel, linux-kernel
Add a function, iov_iter_extract_pages(), to extract a list of pages from
an iterator. The pages may be returned with a reference added or a pin
added or neither, depending on the type of iterator and the direction of
transfer.
The function also indicates the mode of retention that was employed for an
iterator - and therefore how the caller should dispose of the pages later.
There are three cases:
(1) Transfer *into* an ITER_IOVEC or ITER_UBUF iterator.
Extracted pages will have pins obtained on them (but not references)
so that fork() doesn't CoW the pages incorrectly whilst the I/O is in
progress.
The indicated mode of retention will be FOLL_PIN for this case. The
caller should use something like unpin_user_page() to dispose of the
page.
(2) Transfer is *out of* an ITER_IOVEC or ITER_UBUF iterator.
Extracted pages will have references obtained on them, but not pins.
The indicated mode of retention will be FOLL_GET. The caller should
use something like put_page() for page disposal.
(3) Any other sort of iterator.
No refs or pins are obtained on the page, the assumption is made that
the caller will manage page retention.
The indicated mode of retention will be 0. The pages don't need
additional disposal.
Changes:
========
ver #3)
- Switch to using EXPORT_SYMBOL_GPL to prevent indirect 3rd-party access
to get/pin_user_pages_fast()[1].
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Christoph Hellwig <hch@lst.de>
cc: John Hubbard <jhubbard@nvidia.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/Y3zFzdWnWlEJ8X8/@infradead.org/ [1]
Link: https://lore.kernel.org/r/166722777971.2555743.12953624861046741424.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166732025748.3186319.8314014902727092626.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/166869689451.3723671.18242195992447653092.stgit@warthog.procyon.org.uk/ # rfc
---
include/linux/uio.h | 4 +
lib/iov_iter.c | 350 +++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 354 insertions(+)
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 2e3134b14ffd..2fa3ef0f2da3 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -351,4 +351,8 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
};
}
+ssize_t iov_iter_extract_pages(struct iov_iter *i, struct page ***pages,
+ size_t maxsize, unsigned int maxpages,
+ size_t *offset0, unsigned int *cleanup_mode);
+
#endif
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index c3ca28ca68a6..f0f758950a54 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1892,3 +1892,353 @@ void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state)
i->iov -= state->nr_segs - i->nr_segs;
i->nr_segs = state->nr_segs;
}
+
+/*
+ * Extract a list of contiguous pages from an ITER_PIPE iterator. This does
+ * not get references of its own on the pages, nor does it get a pin on them.
+ * If there's a partial page, it adds that first and will then allocate and add
+ * pages into the pipe to make up the buffer space to the amount required.
+ *
+ * The caller must hold the pipe locked and only transferring into a pipe is
+ * supported.
+ */
+static ssize_t iov_iter_extract_pipe_pages(struct iov_iter *i,
+ struct page ***pages, size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ unsigned int nr, offset, chunk, j;
+ struct page **p;
+ size_t left;
+
+ if (!sanity(i))
+ return -EFAULT;
+
+ offset = pipe_npages(i, &nr);
+ if (!nr)
+ return -EFAULT;
+ *offset0 = offset;
+
+ maxpages = min_t(size_t, nr, maxpages);
+ maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+ if (!maxpages)
+ return -ENOMEM;
+ p = *pages;
+
+ left = maxsize;
+ for (j = 0; j < maxpages; j++) {
+ struct page *page = append_pipe(i, left, &offset);
+ if (!page)
+ break;
+ chunk = min_t(size_t, left, PAGE_SIZE - offset);
+ left -= chunk;
+ *p++ = page;
+ }
+ if (!j)
+ return -EFAULT;
+ *cleanup_mode = 0;
+ return maxsize - left;
+}
+
+/*
+ * Extract a list of contiguous pages from an ITER_XARRAY iterator. This does not
+ * get references on the pages, nor does it get a pin on them.
+ */
+static ssize_t iov_iter_extract_xarray_pages(struct iov_iter *i,
+ struct page ***pages, size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ struct page *page, **p;
+ unsigned int nr = 0, offset;
+ loff_t pos = i->xarray_start + i->iov_offset;
+ pgoff_t index = pos >> PAGE_SHIFT;
+ XA_STATE(xas, i->xarray, index);
+
+ offset = pos & ~PAGE_MASK;
+ *offset0 = offset;
+
+ maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+ if (!maxpages)
+ return -ENOMEM;
+ p = *pages;
+
+ rcu_read_lock();
+ for (page = xas_load(&xas); page; page = xas_next(&xas)) {
+ if (xas_retry(&xas, page))
+ continue;
+
+ /* Has the page moved or been split? */
+ if (unlikely(page != xas_reload(&xas))) {
+ xas_reset(&xas);
+ continue;
+ }
+
+ p[nr++] = find_subpage(page, xas.xa_index);
+ if (nr == maxpages)
+ break;
+ }
+ rcu_read_unlock();
+
+ maxsize = min_t(size_t, nr * PAGE_SIZE - offset, maxsize);
+ i->iov_offset += maxsize;
+ i->count -= maxsize;
+ *cleanup_mode = 0;
+ return maxsize;
+}
+
+/*
+ * Extract a list of contiguous pages from an ITER_BVEC iterator. This does
+ * not get references on the pages, nor does it get a pin on them.
+ */
+static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
+ struct page ***pages, size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ struct page **p, *page;
+ size_t skip = i->iov_offset, offset;
+ int k;
+
+ maxsize = min(maxsize, i->bvec->bv_len - skip);
+ skip += i->bvec->bv_offset;
+ page = i->bvec->bv_page + skip / PAGE_SIZE;
+ offset = skip % PAGE_SIZE;
+ *offset0 = offset;
+
+ maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+ if (!maxpages)
+ return -ENOMEM;
+ p = *pages;
+ for (k = 0; k < maxpages; k++)
+ p[k] = page + k;
+
+ maxsize = min_t(size_t, maxsize, maxpages * PAGE_SIZE - offset);
+ i->count -= maxsize;
+ i->iov_offset += maxsize;
+ if (i->iov_offset == i->bvec->bv_len) {
+ i->iov_offset = 0;
+ i->bvec++;
+ i->nr_segs--;
+ }
+ *cleanup_mode = 0;
+ return maxsize;
+}
+
+/*
+ * Get the first segment from an ITER_UBUF or ITER_IOVEC iterator. The
+ * iterator must not be empty.
+ */
+static unsigned long iov_iter_extract_first_user_segment(const struct iov_iter *i,
+ size_t *size)
+{
+ size_t skip;
+ long k;
+
+ if (iter_is_ubuf(i))
+ return (unsigned long)i->ubuf + i->iov_offset;
+
+ for (k = 0, skip = i->iov_offset; k < i->nr_segs; k++, skip = 0) {
+ size_t len = i->iov[k].iov_len - skip;
+
+ if (unlikely(!len))
+ continue;
+ if (*size > len)
+ *size = len;
+ return (unsigned long)i->iov[k].iov_base + skip;
+ }
+ BUG(); // if it had been empty, we wouldn't get called
+}
+
+/*
+ * Extract a list of contiguous pages from a user iterator and get references
+ * on them. This should only be used iff the iterator is user-backed
+ * (IOBUF/UBUF) and data is being transferred out of the buffer described by
+ * the iterator (ie. this is the source).
+ *
+ * The pages are returned with incremented refcounts that the caller must undo
+ * once the transfer is complete, but no additional pins are obtained.
+ *
+ * This is only safe to be used where background IO/DMA is not going to be
+ * modifying the buffer, and so won't cause a problem with CoW on fork.
+ */
+static ssize_t iov_iter_extract_user_pages_and_get(struct iov_iter *i,
+ struct page ***pages,
+ size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ unsigned long addr;
+ unsigned int gup_flags = FOLL_GET;
+ size_t offset;
+ int res;
+
+ if (WARN_ON_ONCE(iov_iter_rw(i) != WRITE))
+ return -EFAULT;
+
+ if (i->nofault)
+ gup_flags |= FOLL_NOFAULT;
+
+ addr = iov_iter_extract_first_user_segment(i, &maxsize);
+ *offset0 = offset = addr % PAGE_SIZE;
+ addr &= PAGE_MASK;
+ maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+ if (!maxpages)
+ return -ENOMEM;
+ res = get_user_pages_fast(addr, maxpages, gup_flags, *pages);
+ if (unlikely(res <= 0))
+ return res;
+ maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset);
+ iov_iter_advance(i, maxsize);
+ *cleanup_mode = FOLL_GET;
+ return maxsize;
+}
+
+/*
+ * Extract a list of contiguous pages from a user iterator and get a pin on
+ * each of them. This should only be used iff the iterator is user-backed
+ * (IOBUF/UBUF) and data is being transferred into the buffer described by the
+ * iterator (ie. this is the destination).
+ *
+ * It does not get refs on the pages, but the pages must be unpinned by the
+ * caller once the transfer is complete.
+ *
+ * This is safe to be used where background IO/DMA *is* going to be modifying
+ * the buffer; using a pin rather than a ref makes sure that CoW happens
+ * correctly in the parent during fork.
+ */
+static ssize_t iov_iter_extract_user_pages_and_pin(struct iov_iter *i,
+ struct page ***pages,
+ size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ unsigned long addr;
+ unsigned int gup_flags = FOLL_PIN | FOLL_WRITE;
+ size_t offset;
+ int res;
+
+ if (WARN_ON_ONCE(iov_iter_rw(i) != READ))
+ return -EFAULT;
+
+ if (i->nofault)
+ gup_flags |= FOLL_NOFAULT;
+
+ addr = first_iovec_segment(i, &maxsize);
+ *offset0 = offset = addr % PAGE_SIZE;
+ addr &= PAGE_MASK;
+ maxpages = want_pages_array(pages, maxsize, offset, maxpages);
+ if (!maxpages)
+ return -ENOMEM;
+ res = pin_user_pages_fast(addr, maxpages, gup_flags, *pages);
+ if (unlikely(res <= 0))
+ return res;
+ maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - offset);
+ iov_iter_advance(i, maxsize);
+ *cleanup_mode = FOLL_PIN;
+ return maxsize;
+}
+
+static ssize_t iov_iter_extract_user_pages(struct iov_iter *i,
+ struct page ***pages, size_t maxsize,
+ unsigned int maxpages,
+ size_t *offset0,
+ unsigned int *cleanup_mode)
+{
+ if (i->data_source)
+ return iov_iter_extract_user_pages_and_get(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+ else
+ return iov_iter_extract_user_pages_and_pin(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+}
+
+/**
+ * iov_iter_extract_pages - Extract a list of contiguous pages from an iterator
+ * @i: The iterator to extract from
+ * @pages: Where to return the list of pages
+ * @maxsize: The maximum amount of iterator to extract
+ * @maxpages: The maximum size of the list of pages
+ * @offset0: Where to return the starting offset into (*@pages)[0]
+ * @cleanup_mode: Where to return the cleanup mode
+ *
+ * Extract a list of contiguous pages from the current point of the iterator,
+ * advancing the iterator. The maximum number of pages and the maximum amount
+ * of page contents can be set.
+ *
+ * If *@pages is NULL, a page list will be allocated to the required size and
+ * *@pages will be set to its base. If *@pages is not NULL, it will be assumed
+ * that the caller allocated a page list at least @maxpages in size and this
+ * will be filled in.
+ *
+ * Extra refs or pins on the pages may be obtained as follows:
+ *
+ * (*) If the iterator is user-backed (ITER_IOVEC/ITER_UBUF) and data is to be
+ * transferred /OUT OF/ the described buffer, refs will be taken on the
+ * pages, but pins will not be added. This can be used for DMA from a
+ * page; it cannot be used for DMA to a page, as it may cause page-COW
+ * problems in fork. *@cleanup_mode will be set to FOLL_GET.
+ *
+ * (*) If the iterator is user-backed (ITER_IOVEC/ITER_UBUF) and data is to be
+ * transferred /INTO/ the described buffer, pins will be added to the
+ * pages, but refs will not be taken. This must be used for DMA to a
+ * page. *@cleanup_mode will be set to FOLL_PIN.
+ *
+ * (*) If the iterator is ITER_PIPE, this must describe a destination for the
+ * data. Additional pages may be allocated and added to the pipe (which
+ * will hold the refs), but neither refs nor pins will be obtained for the
+ * caller. The caller must hold the pipe lock. *@cleanup_mode will be
+ * set to 0.
+ *
+ * (*) If the iterator is ITER_BVEC or ITER_XARRAY, the pages are merely
+ * listed; no extra refs or pins are obtained. *@cleanup_mode will be set
+ * to 0.
+ *
+ * Note also:
+ *
+ * (*) Use with ITER_KVEC is not supported as that may refer to memory that
+ * doesn't have associated page structs.
+ *
+ * (*) Use with ITER_DISCARD is not supported as that has no content.
+ *
+ * On success, the function sets *@pages to the new pagelist, if allocated, and
+ * sets *offset0 to the offset into the first page, *cleanup_mode to the
+ * cleanup required and returns the amount of buffer space added represented by
+ * the page list.
+ *
+ * It may also return -ENOMEM and -EFAULT.
+ */
+ssize_t iov_iter_extract_pages(struct iov_iter *i, struct page ***pages,
+ size_t maxsize, unsigned int maxpages,
+ size_t *offset0, unsigned int *cleanup_mode)
+{
+ maxsize = min_t(size_t, min_t(size_t, maxsize, i->count), MAX_RW_COUNT);
+ if (!maxsize)
+ return 0;
+
+ if (likely(user_backed_iter(i)))
+ return iov_iter_extract_user_pages(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+ if (iov_iter_is_bvec(i))
+ return iov_iter_extract_bvec_pages(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+ if (iov_iter_is_pipe(i))
+ return iov_iter_extract_pipe_pages(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+ if (iov_iter_is_xarray(i))
+ return iov_iter_extract_xarray_pages(i, pages, maxsize,
+ maxpages, offset0,
+ cleanup_mode);
+ return -EFAULT;
+}
+EXPORT_SYMBOL_GPL(iov_iter_extract_pages);
^ permalink raw reply [flat|nested] 6+ messages in thread