From: Hugh Dickins <hughd@xxxxxxxxxx> Subject: 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 There are too many consecutive "if (page"s there in shmem_getpage_gfp(): group it better; and fix the outdated "bring it back from swap" comment.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@xxxxxxxxxx Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/shmem_fs.h | 1 + mm/khugepaged.c | 2 +- mm/shmem.c | 29 +++++++++++++++++------------ 3 files changed, 19 insertions(+), 13 deletions(-) --- a/include/linux/shmem_fs.h~huge-tmpfs-sgp_noalloc-to-stop-collapse_file-on-race +++ a/include/linux/shmem_fs.h @@ -94,6 +94,7 @@ extern unsigned long shmem_partial_swap_ /* Flag allocation requirements to shmem_getpage */ enum sgp_type { SGP_READ, /* don't exceed i_size, don't allocate page */ + SGP_NOALLOC, /* similar, but fail on hole or use fallocated page */ SGP_CACHE, /* don't exceed i_size, may allocate page */ SGP_NOHUGE, /* like SGP_CACHE, but no huge pages */ SGP_HUGE, /* like SGP_CACHE, huge pages preferred */ --- a/mm/khugepaged.c~huge-tmpfs-sgp_noalloc-to-stop-collapse_file-on-race +++ a/mm/khugepaged.c @@ -1721,7 +1721,7 @@ static void collapse_file(struct mm_stru 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; } --- a/mm/shmem.c~huge-tmpfs-sgp_noalloc-to-stop-collapse_file-on-race +++ a/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)) { _
