diff options
| author |   Hugh Dickins <hughd@google.com> | 2021-09-02 14:54:34 -0700 |
|---|---|---|
| committer |   Linus Torvalds <torvalds@linux-foundation.org> | 2021-09-03 09:58:12 -0700 |
| commit | acdd9f8e0fed9f1bd7e83a8ff934694bb4c9a72b (patch) | |
| tree | 4fb4fd74ae8c1086a715d7515a2d7187b20190c5 /mm | |
| parent | huge tmpfs: move shmem_huge_enabled() upwards (diff) | |
| download | linux-dev-acdd9f8e0fed9f1bd7e83a8ff934694bb4c9a72b.tar.xz linux-dev-acdd9f8e0fed9f1bd7e83a8ff934694bb4c9a72b.zip | |
huge tmpfs: SGP_NOALLOC to stop collapse_file() on race
khugepaged's collapse_file() currently uses SGP_NOHUGE to tell
shmem_getpage() not to try allocating a huge page, in the very unlikely
event that a racing hole-punch removes the swapped or fallocated page as
soon as i_pages lock is dropped.
We want to consolidate shmem's huge decisions, removing SGP_HUGE and
SGP_NOHUGE; but cannot quite persuade ourselves that it's okay to regress
the protection in this case - Yang Shi points out that the huge page would
remain indefinitely, charged to root instead of the intended memcg.
collapse_file() should not even allocate a small page in this case: why
proceed if someone is punching a hole? SGP_READ is almost the right flag
here, except that it optimizes away from a fallocated page, with NULL to
tell caller to fill with zeroes (like a hole); whereas collapse_file()'s
sequence relies on using a cache page. Add SGP_NOALLOC just for this.
There are too many consecutive "if (page"s there in shmem_getpage_gfp():
group it better; and fix the outdated "bring it back from swap" comment.
Link: https://lkml.kernel.org/r/1355343b-acf-4653-ef79-6aee40214ac5@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/khugepaged.c | 2 | ||||
| -rw-r--r-- | mm/shmem.c | 29 |
2 files changed, 18 insertions, 13 deletions
diff --git a/mm/khugepaged.c b/mm/khugepaged.c index b0412be08fa2..045cc579f724 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1721,7 +1721,7 @@ static void collapse_file(struct mm_struct *mm, xas_unlock_irq(&xas); /* swap in or instantiate fallocated page */ if (shmem_getpage(mapping->host, index, &page, - SGP_NOHUGE)) { + SGP_NOALLOC)) { result = SCAN_FAIL; goto xa_unlocked; } diff --git a/mm/shmem.c b/mm/shmem.c index 2df6a5370cd7..867cb404090a 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1854,26 +1854,31 @@ repeat: return error; } - if (page) + if (page) { hindex = page->index; - if (page && sgp == SGP_WRITE) - mark_page_accessed(page); - - /* fallocated page? */ - if (page && !PageUptodate(page)) { + if (sgp == SGP_WRITE) + mark_page_accessed(page); + if (PageUptodate(page)) + goto out; + /* fallocated page */ if (sgp != SGP_READ) goto clear; unlock_page(page); put_page(page); - page = NULL; - hindex = index; } - if (page || sgp == SGP_READ) - goto out; /* - * Fast cache lookup did not find it: - * bring it back from swap or allocate. + * SGP_READ: succeed on hole, with NULL page, letting caller zero. + * SGP_NOALLOC: fail on hole, with NULL page, letting caller fail. + */ + *pagep = NULL; + if (sgp == SGP_READ) + return 0; + if (sgp == SGP_NOALLOC) + return -ENOENT; + + /* + * Fast cache lookup and swap lookup did not find it: allocate. */ if (vma && userfaultfd_missing(vma)) { |