| Message ID | 1355343b-acf-4653-ef79-6aee40214ac5@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | huge tmpfs: shmem_is_huge() fixes and cleanups | expand |
On Tue, Aug 17, 2021 at 1:17 AM Hugh Dickins <hughd@google.com> wrote: > > 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 group it better; and fix the outdated "bring it back from swap" comment. > > Signed-off-by: Hugh Dickins <hughd@google.com> Reviewed-by: Yang Shi <shy828301@gmail.com> > --- > include/linux/shmem_fs.h | 1 + > mm/khugepaged.c | 2 +- > mm/shmem.c | 29 +++++++++++++++++------------ > 3 files changed, 19 insertions(+), 13 deletions(-) > > diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h > index 9b7f7ac52351..7d97b15a2f7a 100644 > --- a/include/linux/shmem_fs.h > +++ b/include/linux/shmem_fs.h > @@ -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 */ > 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 740d48ef1eb5..226ac3a911e9 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -1871,26 +1871,31 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index, > 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)) { > -- > 2.26.2 >
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h index 9b7f7ac52351..7d97b15a2f7a 100644 --- a/include/linux/shmem_fs.h +++ b/include/linux/shmem_fs.h @@ -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 */ 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 740d48ef1eb5..226ac3a911e9 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1871,26 +1871,31 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index, 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)) {
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. Signed-off-by: Hugh Dickins <hughd@google.com> --- include/linux/shmem_fs.h | 1 + mm/khugepaged.c | 2 +- mm/shmem.c | 29 +++++++++++++++++------------ 3 files changed, 19 insertions(+), 13 deletions(-)