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huge tmpfs: SGP_NOALLOC to stop collapse_file() on race

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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group it better; and fix the outdated "bring it back from swap" comment.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>
This commit is contained in:
Hugh Dickins 2021-09-02 14:54:34 -07:00 committed by Linus Torvalds
parent c852023e6f
commit acdd9f8e0f
3 changed files with 19 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/shmem_fs.h
View File

@ -94,6 +94,7 @@ extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
/* 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 */

2
mm/khugepaged.c
View File

@ -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;
}

29
mm/shmem.c
View File

@ -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)) {