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Message-ID: <CAPQyPG4GTccLroA2NsdQK_PH1_KB3dD1v3m1FzenCeDW-8qb+g@mail.gmail.com>
Date: Sat, 19 Nov 2011 16:59:10 +0800
From: Nai Xia <nai.xia@...il.com>
To: Mel Gorman <mgorman@...e.de>
Cc: Linux-MM <linux-mm@...ck.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Minchan Kim <minchan.kim@...il.com>, Jan Kara <jack@...e.cz>,
Andy Isaacson <adi@...apodia.org>,
Johannes Weiner <jweiner@...hat.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] mm: compaction: Determine if dirty pages can be
migreated without blocking within ->migratepage
On Sat, Nov 19, 2011 at 12:58 AM, Mel Gorman <mgorman@...e.de> wrote:
> Asynchronous compaction is when allocating transparent hugepages to
> avoid blocking for long periods of time. Due to reports of stalling,
> synchronous compaction is never used but this impacts allocation
> success rates. When deciding whether to migrate dirty pages, the
> following check is made
>
> if (PageDirty(page) && !sync &&
> mapping->a_ops->migratepage != migrate_page)
> rc = -EBUSY;
>
> This skips over all pages using buffer_migrate_page() even though
> it is possible to migrate some of these pages without blocking. This
> patch updates the ->migratepage callback with a "sync" parameter. It
> is the resposibility of the callback to gracefully fail migration of
> the page if it cannot be achieved without blocking.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
> fs/btrfs/disk-io.c | 2 +-
> fs/nfs/internal.h | 2 +-
> fs/nfs/write.c | 4 +-
> include/linux/fs.h | 9 +++-
> include/linux/migrate.h | 2 +-
> mm/migrate.c | 106 ++++++++++++++++++++++++++++++++---------------
> 6 files changed, 83 insertions(+), 42 deletions(-)
>
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 62afe5c..f841f00 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -872,7 +872,7 @@ static int btree_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
>
> #ifdef CONFIG_MIGRATION
> static int btree_migratepage(struct address_space *mapping,
> - struct page *newpage, struct page *page)
> + struct page *newpage, struct page *page, bool sync)
> {
> /*
> * we can't safely write a btree page from here,
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index c1a1bd8..d0c460f 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -328,7 +328,7 @@ void nfs_commit_release_pages(struct nfs_write_data *data);
>
> #ifdef CONFIG_MIGRATION
> extern int nfs_migrate_page(struct address_space *,
> - struct page *, struct page *);
> + struct page *, struct page *, bool);
> #else
> #define nfs_migrate_page NULL
> #endif
> diff --git a/fs/nfs/write.c b/fs/nfs/write.c
> index 1dda78d..33475df 100644
> --- a/fs/nfs/write.c
> +++ b/fs/nfs/write.c
> @@ -1711,7 +1711,7 @@ out_error:
>
> #ifdef CONFIG_MIGRATION
> int nfs_migrate_page(struct address_space *mapping, struct page *newpage,
> - struct page *page)
> + struct page *page, bool sync)
> {
> /*
> * If PagePrivate is set, then the page is currently associated with
> @@ -1726,7 +1726,7 @@ int nfs_migrate_page(struct address_space *mapping, struct page *newpage,
>
> nfs_fscache_release_page(page, GFP_KERNEL);
>
> - return migrate_page(mapping, newpage, page);
> + return migrate_page(mapping, newpage, page, sync);
> }
> #endif
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 0c4df26..67f8e46 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -609,9 +609,12 @@ struct address_space_operations {
> loff_t offset, unsigned long nr_segs);
> int (*get_xip_mem)(struct address_space *, pgoff_t, int,
> void **, unsigned long *);
> - /* migrate the contents of a page to the specified target */
> + /*
> + * migrate the contents of a page to the specified target. If sync
> + * is false, it must not block. If it needs to block, return -EBUSY
> + */
> int (*migratepage) (struct address_space *,
> - struct page *, struct page *);
> + struct page *, struct page *, bool);
> int (*launder_page) (struct page *);
> int (*is_partially_uptodate) (struct page *, read_descriptor_t *,
> unsigned long);
> @@ -2577,7 +2580,7 @@ extern int generic_check_addressable(unsigned, u64);
>
> #ifdef CONFIG_MIGRATION
> extern int buffer_migrate_page(struct address_space *,
> - struct page *, struct page *);
> + struct page *, struct page *, bool);
> #else
> #define buffer_migrate_page NULL
> #endif
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index e39aeec..14e6d2a 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -11,7 +11,7 @@ typedef struct page *new_page_t(struct page *, unsigned long private, int **);
>
> extern void putback_lru_pages(struct list_head *l);
> extern int migrate_page(struct address_space *,
> - struct page *, struct page *);
> + struct page *, struct page *, bool);
> extern int migrate_pages(struct list_head *l, new_page_t x,
> unsigned long private, bool offlining,
> bool sync);
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 578e291..8395697 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -415,7 +415,7 @@ EXPORT_SYMBOL(fail_migrate_page);
> * Pages are locked upon entry and exit.
> */
> int migrate_page(struct address_space *mapping,
> - struct page *newpage, struct page *page)
> + struct page *newpage, struct page *page, bool sync)
> {
> int rc;
>
> @@ -432,19 +432,60 @@ int migrate_page(struct address_space *mapping,
> EXPORT_SYMBOL(migrate_page);
>
> #ifdef CONFIG_BLOCK
> +
> +/* Returns true if all buffers are successfully locked */
> +bool buffer_migrate_lock_buffers(struct buffer_head *head, bool sync)
> +{
> + struct buffer_head *bh = head;
> +
> + /* Simple case, sync compaction */
> + if (sync) {
> + do {
> + get_bh(bh);
> + lock_buffer(bh);
> + bh = bh->b_this_page;
> +
> + } while (bh != head);
> +
> + return true;
> + }
> +
> + /* async case, we cannot block on lock_buffer so use trylock_buffer */
> + do {
> + get_bh(bh);
> + if (!trylock_buffer(bh)) {
> + /*
> + * We failed to lock the buffer and cannot stall in
> + * async migration. Release the taken locks
> + */
> + struct buffer_head *failed_bh = bh;
> + bh = head;
> + do {
> + unlock_buffer(bh);
> + put_bh(bh);
> + bh = bh->b_this_page;
> + } while (bh != failed_bh);
> + return false;
> + }
> +
> + bh = bh->b_this_page;
> + } while (bh != head);
> + return true;
> +}
> +
> /*
> * Migration function for pages with buffers. This function can only be used
> * if the underlying filesystem guarantees that no other references to "page"
> * exist.
> */
> int buffer_migrate_page(struct address_space *mapping,
> - struct page *newpage, struct page *page)
> + struct page *newpage, struct page *page, bool sync)
> {
> struct buffer_head *bh, *head;
> int rc;
>
> if (!page_has_buffers(page))
> - return migrate_page(mapping, newpage, page);
> + return migrate_page(mapping, newpage, page, sync);
>
> head = page_buffers(page);
>
> @@ -453,13 +494,18 @@ int buffer_migrate_page(struct address_space *mapping,
> if (rc)
> return rc;
>
> - bh = head;
> - do {
> - get_bh(bh);
> - lock_buffer(bh);
> - bh = bh->b_this_page;
> -
> - } while (bh != head);
> + if (!buffer_migrate_lock_buffers(head, sync)) {
> + /*
> + * We have to revert the radix tree update. If this returns
> + * non-zero, it either means that the page count changed
> + * which "can't happen" or the slot changed from underneath
> + * us in which case someone operated on a page that did not
> + * have buffers fully migrated which is alarming so warn
> + * that it happened.
> + */
> + WARN_ON(migrate_page_move_mapping(mapping, page, newpage));
> + return -EBUSY;
If this migrate_page_move_mapping() really fails, seems disk IO will be needed
to bring the previously already cached page back, I wonder if we should make the
double check for the two conditions of "page refs is ok " and "all bh
trylocked"
before doing radix_tree_replace_slot() ? which I think does not
involve IO on the
error path.
Nai
> + }
>
> ClearPagePrivate(page);
> set_page_private(newpage, page_private(page));
> @@ -536,10 +582,13 @@ static int writeout(struct address_space *mapping, struct page *page)
> * Default handling if a filesystem does not provide a migration function.
> */
> static int fallback_migrate_page(struct address_space *mapping,
> - struct page *newpage, struct page *page)
> + struct page *newpage, struct page *page, bool sync)
> {
> - if (PageDirty(page))
> + if (PageDirty(page)) {
> + if (!sync)
> + return -EBUSY;
> return writeout(mapping, page);
> + }
>
> /*
> * Buffers may be managed in a filesystem specific way.
> @@ -549,7 +598,7 @@ static int fallback_migrate_page(struct address_space *mapping,
> !try_to_release_page(page, GFP_KERNEL))
> return -EAGAIN;
>
> - return migrate_page(mapping, newpage, page);
> + return migrate_page(mapping, newpage, page, sync);
> }
>
> /*
> @@ -585,29 +634,18 @@ static int move_to_new_page(struct page *newpage, struct page *page,
>
> mapping = page_mapping(page);
> if (!mapping)
> - rc = migrate_page(mapping, newpage, page);
> - else {
> + rc = migrate_page(mapping, newpage, page, sync);
> + else if (mapping->a_ops->migratepage)
> /*
> - * Do not writeback pages if !sync and migratepage is
> - * not pointing to migrate_page() which is nonblocking
> - * (swapcache/tmpfs uses migratepage = migrate_page).
> + * Most pages have a mapping and most filesystems provide a
> + * migratepage callback. Anonymous pages are part of swap
> + * space which also has its own migratepage callback. This
> + * is the most common path for page migration.
> */
> - if (PageDirty(page) && !sync &&
> - mapping->a_ops->migratepage != migrate_page)
> - rc = -EBUSY;
> - else if (mapping->a_ops->migratepage)
> - /*
> - * Most pages have a mapping and most filesystems
> - * should provide a migration function. Anonymous
> - * pages are part of swap space which also has its
> - * own migration function. This is the most common
> - * path for page migration.
> - */
> - rc = mapping->a_ops->migratepage(mapping,
> - newpage, page);
> - else
> - rc = fallback_migrate_page(mapping, newpage, page);
> - }
> + rc = mapping->a_ops->migratepage(mapping,
> + newpage, page, sync);
> + else
> + rc = fallback_migrate_page(mapping, newpage, page, sync);
>
> if (rc) {
> newpage->mapping = NULL;
> --
> 1.7.3.4
>
> --
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