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Shutemov" To: Song Liu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, namit@vmware.com, peterz@infradead.org, oleg@redhat.com, rostedt@goodmis.org, mhiramat@kernel.org, matthew.wilcox@oracle.com, kirill.shutemov@linux.intel.com, kernel-team@fb.com Subject: Re: [PATCH v3 3/5] mm, thp: introduce FOLL_SPLIT_PMD Message-ID: <20190613125718.tgplv5iqkbfhn6vh@box> References: <20190612220320.2223898-1-songliubraving@fb.com> <20190612220320.2223898-4-songliubraving@fb.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190612220320.2223898-4-songliubraving@fb.com> User-Agent: NeoMutt/20180716 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jun 12, 2019 at 03:03:17PM -0700, Song Liu wrote: > This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name says > FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge > page stays as-is. > > FOLL_SPLIT_PMD is useful for cases where we need to use regular pages, > but would switch back to huge page and huge pmd on. One of such example > is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe. > > Signed-off-by: Song Liu > --- > include/linux/mm.h | 1 + > mm/gup.c | 38 +++++++++++++++++++++++++++++++++++--- > 2 files changed, 36 insertions(+), 3 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 0ab8c7d84cd0..e605acc4fc81 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2642,6 +2642,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, > #define FOLL_COW 0x4000 /* internal GUP flag */ > #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 */ > > /* > * NOTE on FOLL_LONGTERM: > diff --git a/mm/gup.c b/mm/gup.c > index ddde097cf9e4..3d05bddb56c9 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -398,7 +398,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > spin_unlock(ptl); > return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); > } > - if (flags & FOLL_SPLIT) { > + if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) { > int ret; > page = pmd_page(*pmd); > if (is_huge_zero_page(page)) { > @@ -407,7 +407,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > split_huge_pmd(vma, pmd, address); > if (pmd_trans_unstable(pmd)) > ret = -EBUSY; > - } else { > + } else if (flags & FOLL_SPLIT) { > if (unlikely(!try_get_page(page))) { > spin_unlock(ptl); > return ERR_PTR(-ENOMEM); > @@ -419,8 +419,40 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma, > put_page(page); > if (pmd_none(*pmd)) > return no_page_table(vma, flags); > - } > + } else { /* flags & FOLL_SPLIT_PMD */ > + unsigned long addr; > + pgprot_t prot; > + pte_t *pte; > + int i; > + > + spin_unlock(ptl); > + split_huge_pmd(vma, pmd, address); All the code below is only relevant for file-backed THP. It will break for anon-THP. And I'm not convinced that it belongs here at all. User requested PMD split and it is done after split_huge_pmd(). The rest can be handled by the caller as needed. > + lock_page(page); > + pte = get_locked_pte(mm, address, &ptl); > + if (!pte) { > + unlock_page(page); > + return no_page_table(vma, flags); Or should it be -ENOMEM? > + } > > + /* get refcount for every small page */ > + page_ref_add(page, HPAGE_PMD_NR); > + > + prot = READ_ONCE(vma->vm_page_prot); > + for (i = 0, addr = address & PMD_MASK; > + i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) { > + struct page *p = page + i; > + > + pte = pte_offset_map(pmd, addr); > + VM_BUG_ON(!pte_none(*pte)); > + set_pte_at(mm, addr, pte, mk_pte(p, prot)); > + page_add_file_rmap(p, false); > + } > + > + spin_unlock(ptl); > + unlock_page(page); > + add_mm_counter(mm, mm_counter_file(page), HPAGE_PMD_NR); > + ret = 0; > + } > return ret ? ERR_PTR(ret) : > follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); > } > -- > 2.17.1 > -- Kirill A. Shutemov