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Thu, 09 Jun 2022 14:35:08 +0000 Date: Thu, 9 Jun 2022 15:35:08 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, linux-aio@kvack.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-mm@kvack.org, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-ntfs-dev@lists.sourceforge.net, ocfs2-devel@oss.oracle.com, linux-mtd@lists.infradead.org, virtualization@lists.linux-foundation.org, Christoph Hellwig Subject: Re: [PATCH v2 03/19] fs: Add aops->migrate_folio Message-ID: References: <20220608150249.3033815-1-willy@infradead.org> <20220608150249.3033815-4-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1654785321; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=TA+NCKNN+eR/98pPS94dU7ufUL4m7Zs6MV61a+jrgiQ=; b=C5JEB7BgahmEek5wn6OX3TzkbcC2TrSYoO7rz5W0H+VskyhOlgY9o6z4mMmVP6v/Ak9V1V hK4IMyxsdCA8Q2kOUxU9TnFKXsNiahQ7NShnzIVsXJvIY5feuxdhp7qh0EjZvg3cVkkWaU mAiyu7g6b4SekaAISm5ICEsKne0yyV0= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1654785321; a=rsa-sha256; cv=none; b=SaZV7jNDIbdQW2h3OlsGVz/GcTagcFWIPeYvc7IDTXtS1ABu+09Sxn4YAB09y06EGRaa2P FLLE3r7AD5LJDxgTKWQ43P3N0fpJPm7MRHMlSsgPu63B9Zcne2jPmnSfTsGu+bkjkcOoJ5 rEXZ0cEVodOhfJ7ocAZ3Zxqy79baaPU= ARC-Authentication-Results: i=1; imf29.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=mJcV7mgb; dmarc=none; spf=none (imf29.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Rspamd-Server: rspam11 X-Rspam-User: Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=mJcV7mgb; dmarc=none; spf=none (imf29.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Stat-Signature: 5qaenmwpk79cfzffjd3umfz7qh8o9sqi X-Rspamd-Queue-Id: 9D6EA120078 X-HE-Tag: 1654785320-439783 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 Thu, Jun 09, 2022 at 02:50:20PM +0200, David Hildenbrand wrote: > On 08.06.22 17:02, Matthew Wilcox (Oracle) wrote: > > diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst > > index c0fe711f14d3..3d28b23676bd 100644 > > --- a/Documentation/filesystems/locking.rst > > +++ b/Documentation/filesystems/locking.rst > > @@ -253,7 +253,8 @@ prototypes:: > > void (*free_folio)(struct folio *); > > int (*direct_IO)(struct kiocb *, struct iov_iter *iter); > > bool (*isolate_page) (struct page *, isolate_mode_t); > > - int (*migratepage)(struct address_space *, struct page *, struct page *); > > + int (*migrate_folio)(struct address_space *, struct folio *dst, > > + struct folio *src, enum migrate_mode); > > void (*putback_page) (struct page *); > > isolate_page/putback_page are leftovers from the previous patch, no? Argh, right, I completely forgot I needed to update the documentation in that patch. > > +++ b/Documentation/vm/page_migration.rst > > @@ -181,22 +181,23 @@ which are function pointers of struct address_space_operations. > > Once page is successfully isolated, VM uses page.lru fields so driver > > shouldn't expect to preserve values in those fields. > > > > -2. ``int (*migratepage) (struct address_space *mapping,`` > > -| ``struct page *newpage, struct page *oldpage, enum migrate_mode);`` > > - > > - After isolation, VM calls migratepage() of driver with the isolated page. > > - The function of migratepage() is to move the contents of the old page to the > > - new page > > - and set up fields of struct page newpage. Keep in mind that you should > > - indicate to the VM the oldpage is no longer movable via __ClearPageMovable() > > - under page_lock if you migrated the oldpage successfully and returned > > - MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver > > - can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time > > - because VM interprets -EAGAIN as "temporary migration failure". On returning > > - any error except -EAGAIN, VM will give up the page migration without > > - retrying. > > - > > - Driver shouldn't touch the page.lru field while in the migratepage() function. > > +2. ``int (*migrate_folio) (struct address_space *mapping,`` > > +| ``struct folio *dst, struct folio *src, enum migrate_mode);`` > > + > > + After isolation, VM calls the driver's migrate_folio() with the > > + isolated folio. The purpose of migrate_folio() is to move the contents > > + of the source folio to the destination folio and set up the fields > > + of destination folio. Keep in mind that you should indicate to the > > + VM the source folio is no longer movable via __ClearPageMovable() > > + under folio if you migrated the source successfully and returned > > + MIGRATEPAGE_SUCCESS. If driver cannot migrate the folio at the > > + moment, driver can return -EAGAIN. On -EAGAIN, VM will retry folio > > + migration in a short time because VM interprets -EAGAIN as "temporary > > + migration failure". On returning any error except -EAGAIN, VM will > > + give up the folio migration without retrying. > > + > > + Driver shouldn't touch the folio.lru field while in the migrate_folio() > > + function. > > > > 3. ``void (*putback_page)(struct page *);`` > > Hmm, here it's a bit more complicated now, because we essentially have > two paths: LRU+migrate_folio or !LRU+movable_ops > (isolate/migrate/putback page) Oh ... actually, this is just documenting the driver side of things. I don't really like how it's written. Here, have some rewritten documentation (which is now part of the previous patch): +++ b/Documentation/vm/page_migration.rst @@ -152,110 +152,15 @@ Steps: Non-LRU page migration ====================== -Although migration originally aimed for reducing the latency of memory accesses -for NUMA, compaction also uses migration to create high-order pages. +Although migration originally aimed for reducing the latency of memory +accesses for NUMA, compaction also uses migration to create high-order +pages. For compaction purposes, it is also useful to be able to move +non-LRU pages, such as zsmalloc and virtio-balloon pages. -Current problem of the implementation is that it is designed to migrate only -*LRU* pages. However, there are potential non-LRU pages which can be migrated -in drivers, for example, zsmalloc, virtio-balloon pages. - -For virtio-balloon pages, some parts of migration code path have been hooked -up and added virtio-balloon specific functions to intercept migration logics. -It's too specific to a driver so other drivers who want to make their pages -movable would have to add their own specific hooks in the migration path. - -To overcome the problem, VM supports non-LRU page migration which provides -generic functions for non-LRU movable pages without driver specific hooks -in the migration path. - -If a driver wants to make its pages movable, it should define three functions -which are function pointers of struct address_space_operations. - -1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);`` - - What VM expects from isolate_page() function of driver is to return *true* - if driver isolates the page successfully. On returning true, VM marks the page - as PG_isolated so concurrent isolation in several CPUs skip the page - for isolation. If a driver cannot isolate the page, it should return *false*. - - Once page is successfully isolated, VM uses page.lru fields so driver - shouldn't expect to preserve values in those fields. - -2. ``int (*migratepage) (struct address_space *mapping,`` -| ``struct page *newpage, struct page *oldpage, enum migrate_mode);`` - - After isolation, VM calls migratepage() of driver with the isolated page. - The function of migratepage() is to move the contents of the old page to the - new page - and set up fields of struct page newpage. Keep in mind that you should - indicate to the VM the oldpage is no longer movable via __ClearPageMovable() - under page_lock if you migrated the oldpage successfully and returned - MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver - can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time - because VM interprets -EAGAIN as "temporary migration failure". On returning - any error except -EAGAIN, VM will give up the page migration without - retrying. - - Driver shouldn't touch the page.lru field while in the migratepage() function. - -3. ``void (*putback_page)(struct page *);`` - - If migration fails on the isolated page, VM should return the isolated page - to the driver so VM calls the driver's putback_page() with the isolated page. - In this function, the driver should put the isolated page back into its own data - structure. - -Non-LRU movable page flags - - There are two page flags for supporting non-LRU movable page. - - * PG_movable - - Driver should use the function below to make page movable under page_lock:: - - void __SetPageMovable(struct page *page, struct address_space *mapping) - - It needs argument of address_space for registering migration - family functions which will be called by VM. Exactly speaking, - PG_movable is not a real flag of struct page. Rather, VM - reuses the page->mapping's lower bits to represent it:: - - #define PAGE_MAPPING_MOVABLE 0x2 - page->mapping = page->mapping | PAGE_MAPPING_MOVABLE; - - so driver shouldn't access page->mapping directly. Instead, driver should - use page_mapping() which masks off the low two bits of page->mapping under - page lock so it can get the right struct address_space. - - For testing of non-LRU movable pages, VM supports __PageMovable() function. - However, it doesn't guarantee to identify non-LRU movable pages because - the page->mapping field is unified with other variables in struct page. - If the driver releases the page after isolation by VM, page->mapping - doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set - (look at __ClearPageMovable). But __PageMovable() is cheap to call whether - page is LRU or non-LRU movable once the page has been isolated because LRU - pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also - good for just peeking to test non-LRU movable pages before more expensive - checking with lock_page() in pfn scanning to select a victim. - - For guaranteeing non-LRU movable page, VM provides PageMovable() function. - Unlike __PageMovable(), PageMovable() validates page->mapping and - mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents - sudden destroying of page->mapping. - - Drivers using __SetPageMovable() should clear the flag via - __ClearMovablePage() under page_lock() before the releasing the page. - - * PG_isolated - - To prevent concurrent isolation among several CPUs, VM marks isolated page - as PG_isolated under lock_page(). So if a CPU encounters PG_isolated - non-LRU movable page, it can skip it. Driver doesn't need to manipulate the - flag because VM will set/clear it automatically. Keep in mind that if the - driver sees a PG_isolated page, it means the page has been isolated by the - VM so it shouldn't touch the page.lru field. - The PG_isolated flag is aliased with the PG_reclaim flag so drivers - shouldn't use PG_isolated for its own purposes. +If a driver wants to make its pages movable, it should define a struct +movable_operations. It then needs to call __SetPageMovable() on each +page that it may be able to move. This uses the ``page->mapping`` field, +so this field is not available for the driver to use for other purposes. Monitoring Migration ===================== @@ -286,3 +191,5 @@ THP_MIGRATION_FAIL and PGMIGRATE_FAIL to increase. Christoph Lameter, May 8, 2006. Minchan Kim, Mar 28, 2016. + +.. kernel-doc:: include/linux/migrate.h +++ b/include/linux/migrate.h @@ -19,6 +19,43 @@ struct migration_target_control; */ #define MIGRATEPAGE_SUCCESS 0 +/** + * struct movable_operations - Driver page migration + * @isolate_page: + * The VM calls this function to prepare the page to be moved. The page + * is locked and the driver should not unlock it. The driver should + * return ``true`` if the page is movable and ``false`` if it is not + * currently movable. After this function returns, the VM uses the + * page->lru field, so the driver must preserve any information which + * is usually stored here. + * + * @migrate_page: + * After isolation, the VM calls this function with the isolated + * @src page. The driver should copy the contents of the + * @src page to the @dst page and set up the fields of @dst page. + * Both pages are locked. + * If page migration is successful, the driver should call + * __ClearPageMovable(@src) and return MIGRATEPAGE_SUCCESS. + * If the driver cannot migrate the page at the moment, it can return + * -EAGAIN. The VM interprets this as a temporary migration failure and + * will retry it later. Any other error value is a permanent migration + * failure and migration will not be retried. + * The driver shouldn't touch the @src->lru field while in the + * migrate_page() function. It may write to @dst->lru. + * + * @putback_page: + * If migration fails on the isolated page, the VM informs the driver + * that the page is no longer a candidate for migration by calling + * this function. The driver should put the isolated page back into + * its own data structure. + */ +struct movable_operations { + bool (*isolate_page)(struct page *, isolate_mode_t); + int (*migrate_page)(struct page *dst, struct page *src, + enum migrate_mode); + void (*putback_page)(struct page *); +}; + /* Defined in mm/debug.c: */ extern const char *migrate_reason_names[MR_TYPES];