From: Hugh Dickins hughd@google.com
mainline inclusion from mainline-v5.18-rc1 commit 56a8c8eb1eaf21261be8cdc4e3715239ac087342 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I6113U CVE: NA
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=...
--------------------------------
Mikulas asked in "Do we still need commit a0ee5ec520ed ('tmpfs: allocate on read when stacked')?" in [1]
Lukas noticed this unusual behavior of loop device backed by tmpfs in [2].
Normally, shmem_file_read_iter() copies the ZERO_PAGE when reading holes; but if it looks like it might be a read for "a stacking filesystem", it allocates actual pages to the page cache, and even marks them as dirty. And reads from the loop device do satisfy the test that is used.
This oddity was added for an old version of unionfs, to help to limit its usage to the limited size of the tmpfs mount involved; but about the same time as the tmpfs mod went in (2.6.25), unionfs was reworked to proceed differently; and the mod kept just in case others needed it.
Do we still need it? I cannot answer with more certainty than "Probably not". It's nasty enough that we really should try to delete it; but if a regression is reported somewhere, then we might have to revert later.
It's not quite as simple as just removing the test (as Mikulas did): xfstests generic/013 hung because splice from tmpfs failed on page not up-to-date and page mapping unset. That can be fixed just by marking the ZERO_PAGE as Uptodate, which of course it is: do so in pagecache_init() - it might be useful to others than tmpfs.
My intention, though, was to stop using the ZERO_PAGE here altogether: surely iov_iter_zero() is better for this case? Sadly not: it relies on clear_user(), and the x86 clear_user() is slower than its copy_user() [3].
But while we are still using the ZERO_PAGE, let's stop dirtying its struct page cacheline with unnecessary get_page() and put_page().
Link: https://lore.kernel.org/linux-mm/alpine.LRH.2.02.2007210510230.6959@file01.i... [1] Link: https://lore.kernel.org/linux-mm/20211126075100.gd64odg2bcptiqeb@work/ [2] Link: https://lore.kernel.org/lkml/2f5ca5e4-e250-a41c-11fb-a7f4ebc7e1c9@google.com... [3] Link: https://lkml.kernel.org/r/90bc5e69-9984-b5fa-a685-be55f2b64b@google.com Signed-off-by: Hugh Dickins hughd@google.com Reported-by: Mikulas Patocka mpatocka@redhat.com Reported-by: Lukas Czerner lczerner@redhat.com Acked-by: Darrick J. Wong djwong@kernel.org Reviewed-by: Christoph Hellwig hch@lst.de Cc: Zdenek Kabelac zkabelac@redhat.com Cc: "Darrick J. Wong" djwong@kernel.org Cc: Miklos Szeredi miklos@szeredi.hu Cc: Borislav Petkov bp@suse.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Ma Wupeng mawupeng1@huawei.com Reviewed-by: tong tiangen tongtiangen@huawei.com Signed-off-by: Zheng Zengkai zhengzengkai@huawei.com Signed-off-by: Jialin Zhang zhangjialin11@huawei.com --- mm/filemap.c | 6 ++++++ mm/shmem.c | 20 ++++++-------------- 2 files changed, 12 insertions(+), 14 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c index 98b448d9873f..a44baa79c1e7 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -996,6 +996,12 @@ void __init pagecache_init(void) init_waitqueue_head(&page_wait_table[i]);
page_writeback_init(); + + /* + * tmpfs uses the ZERO_PAGE for reading holes: it is up-to-date, + * and splice's page_cache_pipe_buf_confirm() needs to see that. + */ + SetPageUptodate(ZERO_PAGE(0)); }
/* diff --git a/mm/shmem.c b/mm/shmem.c index ad2d68150ed2..c5028634afee 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2573,19 +2573,10 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) struct address_space *mapping = inode->i_mapping; pgoff_t index; unsigned long offset; - enum sgp_type sgp = SGP_READ; int error = 0; ssize_t retval = 0; loff_t *ppos = &iocb->ki_pos;
- /* - * Might this read be for a stacking filesystem? Then when reading - * holes of a sparse file, we actually need to allocate those pages, - * and even mark them dirty, so it cannot exceed the max_blocks limit. - */ - if (!iter_is_iovec(to)) - sgp = SGP_CACHE; - index = *ppos >> PAGE_SHIFT; offset = *ppos & ~PAGE_MASK;
@@ -2594,6 +2585,7 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) pgoff_t end_index; unsigned long nr, ret; loff_t i_size = i_size_read(inode); + bool got_page;
end_index = i_size >> PAGE_SHIFT; if (index > end_index) @@ -2604,15 +2596,13 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) break; }
- error = shmem_getpage(inode, index, &page, sgp); + error = shmem_getpage(inode, index, &page, SGP_READ); if (error) { if (error == -EINVAL) error = 0; break; } if (page) { - if (sgp == SGP_CACHE) - set_page_dirty(page); unlock_page(page); }
@@ -2646,9 +2636,10 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) */ if (!offset) mark_page_accessed(page); + got_page = true; } else { page = ZERO_PAGE(0); - get_page(page); + got_page = false; }
/* @@ -2661,7 +2652,8 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to) index += offset >> PAGE_SHIFT; offset &= ~PAGE_MASK;
- put_page(page); + if (got_page) + put_page(page); if (!iov_iter_count(to)) break; if (ret < nr) {