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Date: Tue, 8 Dec 2020 16:15:59 +0800
From: Chao Yu <yuchao0@...wei.com>
To: Gao Xiang <hsiangkao@...hat.com>, <linux-erofs@...ts.ozlabs.org>
CC: LKML <linux-kernel@...r.kernel.org>, Chao Yu <chao@...nel.org>
Subject: Re: [PATCH v2 1/3] erofs: get rid of magical Z_EROFS_MAPPING_STAGING
On 2020/12/7 9:23, Gao Xiang wrote:
> Previously, we played around with magical page->mapping for short-lived
> temporary pages since we need to identify different types of pages in
> the same pcluster but both invalidated and short-lived temporary pages
> can have page->mapping == NULL. It was considered as safe because that
> temporary pages are all non-LRU / non-movable pages.
>
> This patch tends to use specific page->private to identify short-lived
> pages instead so it won't rely on page->mapping anymore. Details are
> described in "compress.h" as well.
>
> Signed-off-by: Gao Xiang <hsiangkao@...hat.com>
> ---
> tested with ro_fsstress for a whole night.
>
> The old "[PATCH 4/4] erofs: complete a missing case for inplace I/O" is
> temporarily dropped since ro_fsstress failed with such modification,
> will look into later.
>
> fs/erofs/compress.h | 50 ++++++++++++++++++++++++++++++-----------
> fs/erofs/decompressor.c | 2 +-
> fs/erofs/zdata.c | 42 +++++++++++++++++++++-------------
> fs/erofs/zdata.h | 1 +
> 4 files changed, 65 insertions(+), 30 deletions(-)
>
> diff --git a/fs/erofs/compress.h b/fs/erofs/compress.h
> index 3d452443c545..2bbf47f353ef 100644
> --- a/fs/erofs/compress.h
> +++ b/fs/erofs/compress.h
> @@ -26,30 +26,54 @@ struct z_erofs_decompress_req {
> bool inplace_io, partial_decoding;
> };
>
> +#define Z_EROFS_SHORTLIVED_PAGE (-1UL << 2)
> +
> /*
> - * - 0x5A110C8D ('sallocated', Z_EROFS_MAPPING_STAGING) -
> - * used to mark temporary allocated pages from other
> - * file/cached pages and NULL mapping pages.
> + * For all pages in a pcluster, page->private should be one of
> + * Type Last 2bits page->private
> + * short-lived page 00 Z_EROFS_SHORTLIVED_PAGE
> + * cached/managed page 00 pointer to z_erofs_pcluster
> + * online page (file-backed, 01/10/11 sub-index << 2 | count
> + * some pages can be used for inplace I/O)
> + *
> + * page->mapping should be one of
> + * Type page->mapping
> + * short-lived page NULL
> + * cached/managed page non-NULL or NULL (invalidated/truncated page)
> + * online page non-NULL
> + *
> + * For all managed pages, PG_private should be set with 1 extra refcount,
> + * which is used for page reclaim / migration.
FYI, there is a generic way to set/clear page_private, it binds the private
value set and page count operation in one function:
attach_page_private()
detach_page_private()
If there are use cases, let's try to use them as much as possible.
> */
> -#define Z_EROFS_MAPPING_STAGING ((void *)0x5A110C8D)
>
> -/* check if a page is marked as staging */
> -static inline bool z_erofs_page_is_staging(struct page *page)
> +/*
> + * short-lived pages are pages directly from buddy system with specific
> + * page->private (no need to set PagePrivate since these are non-LRU /
> + * non-movable pages and bypass reclaim / migration code).
> + */
> +static inline bool z_erofs_is_shortlived_page(struct page *page)
> {
> - return page->mapping == Z_EROFS_MAPPING_STAGING;
> + if (page->private != Z_EROFS_SHORTLIVED_PAGE)
> + return false;
> +
> + DBG_BUGON(page->mapping);
> + return true;
> }
>
> -static inline bool z_erofs_put_stagingpage(struct list_head *pagepool,
> - struct page *page)
> +static inline bool z_erofs_put_shortlivedpage(struct list_head *pagepool,
> + struct page *page)
> {
> - if (!z_erofs_page_is_staging(page))
> + if (!z_erofs_is_shortlived_page(page))
> return false;
>
> - /* staging pages should not be used by others at the same time */
> - if (page_ref_count(page) > 1)
> + /* short-lived pages should not be used by others at the same time */
> + if (page_ref_count(page) > 1) {
Does this be a possible case?
> put_page(page);
> - else
> + } else {
> + /* follow the pcluster rule above. */
> + set_page_private(page, 0);
> list_add(&page->lru, pagepool);
> + }
> return true;
> }
>
> diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
> index cbadbf55c6c2..1cb1ffd10569 100644
> --- a/fs/erofs/decompressor.c
> +++ b/fs/erofs/decompressor.c
> @@ -76,7 +76,7 @@ static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
> victim = erofs_allocpage(pagepool, GFP_KERNEL);
> if (!victim)
> return -ENOMEM;
> - victim->mapping = Z_EROFS_MAPPING_STAGING;
> + set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
> }
> rq->out[i] = victim;
> }
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 86fd3bf62af6..afeadf413c2c 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -255,6 +255,7 @@ int erofs_try_to_free_cached_page(struct address_space *mapping,
> erofs_workgroup_unfreeze(&pcl->obj, 1);
>
> if (ret) {
> + set_page_private(page, 0);
> ClearPagePrivate(page);
> put_page(page);
detach_page_private()?
Thanks,
> }
> @@ -648,12 +649,12 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>
> retry:
> err = z_erofs_attach_page(clt, page, page_type);
> - /* should allocate an additional staging page for pagevec */
> + /* should allocate an additional short-lived page for pagevec */
> if (err == -EAGAIN) {
> struct page *const newpage =
> alloc_page(GFP_NOFS | __GFP_NOFAIL);
>
> - newpage->mapping = Z_EROFS_MAPPING_STAGING;
> + set_page_private(newpage, Z_EROFS_SHORTLIVED_PAGE);
> err = z_erofs_attach_page(clt, newpage,
> Z_EROFS_PAGE_TYPE_EXCLUSIVE);
> if (!err)
> @@ -710,6 +711,11 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
> queue_work(z_erofs_workqueue, &io->u.work);
> }
>
> +static bool z_erofs_page_is_invalidated(struct page *page)
> +{
> + return !page->mapping && !z_erofs_is_shortlived_page(page);
> +}
> +
> static void z_erofs_decompressqueue_endio(struct bio *bio)
> {
> tagptr1_t t = tagptr_init(tagptr1_t, bio->bi_private);
> @@ -722,7 +728,7 @@ static void z_erofs_decompressqueue_endio(struct bio *bio)
> struct page *page = bvec->bv_page;
>
> DBG_BUGON(PageUptodate(page));
> - DBG_BUGON(!page->mapping);
> + DBG_BUGON(z_erofs_page_is_invalidated(page));
>
> if (err)
> SetPageError(page);
> @@ -795,9 +801,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
>
> /* all pages in pagevec ought to be valid */
> DBG_BUGON(!page);
> - DBG_BUGON(!page->mapping);
> + DBG_BUGON(z_erofs_page_is_invalidated(page));
>
> - if (z_erofs_put_stagingpage(pagepool, page))
> + if (z_erofs_put_shortlivedpage(pagepool, page))
> continue;
>
> if (page_type == Z_EROFS_VLE_PAGE_TYPE_HEAD)
> @@ -831,9 +837,9 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
>
> /* all compressed pages ought to be valid */
> DBG_BUGON(!page);
> - DBG_BUGON(!page->mapping);
> + DBG_BUGON(z_erofs_page_is_invalidated(page));
>
> - if (!z_erofs_page_is_staging(page)) {
> + if (!z_erofs_is_shortlived_page(page)) {
> if (erofs_page_is_managed(sbi, page)) {
> if (!PageUptodate(page))
> err = -EIO;
> @@ -858,7 +864,7 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
> overlapped = true;
> }
>
> - /* PG_error needs checking for inplaced and staging pages */
> + /* PG_error needs checking for all non-managed pages */
> if (PageError(page)) {
> DBG_BUGON(PageUptodate(page));
> err = -EIO;
> @@ -897,8 +903,8 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
> if (erofs_page_is_managed(sbi, page))
> continue;
>
> - /* recycle all individual staging pages */
> - (void)z_erofs_put_stagingpage(pagepool, page);
> + /* recycle all individual short-lived pages */
> + (void)z_erofs_put_shortlivedpage(pagepool, page);
>
> WRITE_ONCE(compressed_pages[i], NULL);
> }
> @@ -908,10 +914,10 @@ static int z_erofs_decompress_pcluster(struct super_block *sb,
> if (!page)
> continue;
>
> - DBG_BUGON(!page->mapping);
> + DBG_BUGON(z_erofs_page_is_invalidated(page));
>
> - /* recycle all individual staging pages */
> - if (z_erofs_put_stagingpage(pagepool, page))
> + /* recycle all individual short-lived pages */
> + if (z_erofs_put_shortlivedpage(pagepool, page))
> continue;
>
> if (err < 0)
> @@ -1011,13 +1017,17 @@ static struct page *pickup_page_for_submission(struct z_erofs_pcluster *pcl,
> mapping = READ_ONCE(page->mapping);
>
> /*
> - * unmanaged (file) pages are all locked solidly,
> + * file-backed online pages in plcuster are all locked steady,
> * therefore it is impossible for `mapping' to be NULL.
> */
> if (mapping && mapping != mc)
> /* ought to be unmanaged pages */
> goto out;
>
> + /* directly return for shortlived page as well */
> + if (z_erofs_is_shortlived_page(page))
> + goto out;
> +
> lock_page(page);
>
> /* only true if page reclaim goes wrong, should never happen */
> @@ -1062,8 +1072,8 @@ static struct page *pickup_page_for_submission(struct z_erofs_pcluster *pcl,
> out_allocpage:
> page = erofs_allocpage(pagepool, gfp | __GFP_NOFAIL);
> if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
> - /* non-LRU / non-movable temporary page is needed */
> - page->mapping = Z_EROFS_MAPPING_STAGING;
> + /* turn into temporary page if fails */
> + set_page_private(page, Z_EROFS_SHORTLIVED_PAGE);
> tocache = false;
> }
>
> diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h
> index 68c9b29fc0ca..b503b353d4ab 100644
> --- a/fs/erofs/zdata.h
> +++ b/fs/erofs/zdata.h
> @@ -173,6 +173,7 @@ static inline void z_erofs_onlinepage_endio(struct page *page)
>
> v = atomic_dec_return(u.o);
> if (!(v & Z_EROFS_ONLINEPAGE_COUNT_MASK)) {
> + set_page_private(page, 0);
> ClearPagePrivate(page);
> if (!PageError(page))
> SetPageUptodate(page);
>
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