Instrumenting filemap_nr_thps_inc() should be sufficient for ensuring writable file mappings will not be THP-backed. If filemap_nr_thps_dec() in unaccount_page_cache_page() and filemap_nr_thps() in do_dentry_open() race, the worst case is an unnecessary truncation. We could introduce a memory barrier in unaccount_page_cache_page(), but I'm unsure it would significantly mitigate the risk of spurious truncations. Barring synchronization between do_dentry_open() and ongoing page cache operations, there could be an in-progress delete_from_page_cache_batch() that has not yet updated accounting for THPs in its targeted range. -- Collin On Mon, Apr 5, 2021 at 7:05 PM William Kucharski <william.kucharski@xxxxxxxxxx> wrote: > > > I saw a similar change a few years ago with my prototype: > > https://lore.kernel.org/linux-mm/5BB682E1-DD52-4AA9-83E9-DEF091E0C709@xxxxxxxxxx/ > > the key being a very nice drop in iTLB-load-misses, so it looks like your code is > having the right effect. > > What about the call to filemap_nr_thps_dec() in unaccount_page_cache_page() - does > that need an smp_mb() as well? > > -- Bill > > > On Apr 5, 2021, at 6:15 PM, Collin Fijalkovich <cfijalkovich@xxxxxxxxxx> wrote: > > > > v2 has been uploaded with performance testing results: > > https://lore.kernel.org/patchwork/patch/1408266/ > > > > > > > > On Mon, Mar 22, 2021 at 2:59 PM Collin Fijalkovich > > <cfijalkovich@xxxxxxxxxx> wrote: > >> > >> Transparent huge pages are supported for read-only non-shmem filesystems, > >> but are only used for vmas with VM_DENYWRITE. This condition ensures that > >> file THPs are protected from writes while an application is running > >> (ETXTBSY). Any existing file THPs are then dropped from the page cache > >> when a file is opened for write in do_dentry_open(). Since sys_mmap > >> ignores MAP_DENYWRITE, this constrains the use of file THPs to vmas > >> produced by execve(). > >> > >> Systems that make heavy use of shared libraries (e.g. Android) are unable > >> to apply VM_DENYWRITE through the dynamic linker, preventing them from > >> benefiting from the resultant reduced contention on the TLB. > >> > >> This patch reduces the constraint on file THPs allowing use with any > >> executable mapping from a file not opened for write (see > >> inode_is_open_for_write()). It also introduces additional conditions to > >> ensure that files opened for write will never be backed by file THPs. > >> > >> Restricting the use of THPs to executable mappings eliminates the risk that > >> a read-only file later opened for write would encounter significant > >> latencies due to page cache truncation. > >> > >> The ld linker flag '-z max-page-size=(hugepage size)' can be used to > >> produce executables with the necessary layout. The dynamic linker must > >> map these file's segments at a hugepage size aligned vma for the mapping to > >> be backed with THPs. > >> > >> Signed-off-by: Collin Fijalkovich <cfijalkovich@xxxxxxxxxx> > >> --- > >> fs/open.c | 13 +++++++++++-- > >> mm/khugepaged.c | 16 +++++++++++++++- > >> 2 files changed, 26 insertions(+), 3 deletions(-) > >> > >> diff --git a/fs/open.c b/fs/open.c > >> index e53af13b5835..f76e960d10ea 100644 > >> --- a/fs/open.c > >> +++ b/fs/open.c > >> @@ -852,8 +852,17 @@ static int do_dentry_open(struct file *f, > >> * XXX: Huge page cache doesn't support writing yet. Drop all page > >> * cache for this file before processing writes. > >> */ > >> - if ((f->f_mode & FMODE_WRITE) && filemap_nr_thps(inode->i_mapping)) > >> - truncate_pagecache(inode, 0); > >> + if (f->f_mode & FMODE_WRITE) { > >> + /* > >> + * Paired with smp_mb() in collapse_file() to ensure nr_thps > >> + * is up to date and the update to i_writecount by > >> + * get_write_access() is visible. Ensures subsequent insertion > >> + * of THPs into the page cache will fail. > >> + */ > >> + smp_mb(); > >> + if (filemap_nr_thps(inode->i_mapping)) > >> + truncate_pagecache(inode, 0); > >> + } > >> > >> return 0; > >> > >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >> index a7d6cb912b05..4c7cc877d5e3 100644 > >> --- a/mm/khugepaged.c > >> +++ b/mm/khugepaged.c > >> @@ -459,7 +459,8 @@ static bool hugepage_vma_check(struct vm_area_struct *vma, > >> > >> /* Read-only file mappings need to be aligned for THP to work. */ > >> if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && vma->vm_file && > >> - (vm_flags & VM_DENYWRITE)) { > >> + !inode_is_open_for_write(vma->vm_file->f_inode) && > >> + (vm_flags & VM_EXEC)) { > >> return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff, > >> HPAGE_PMD_NR); > >> } > >> @@ -1872,6 +1873,19 @@ static void collapse_file(struct mm_struct *mm, > >> else { > >> __mod_lruvec_page_state(new_page, NR_FILE_THPS, nr); > >> filemap_nr_thps_inc(mapping); > >> + /* > >> + * Paired with smp_mb() in do_dentry_open() to ensure > >> + * i_writecount is up to date and the update to nr_thps is > >> + * visible. Ensures the page cache will be truncated if the > >> + * file is opened writable. > >> + */ > >> + smp_mb(); > >> + if (inode_is_open_for_write(mapping->host)) { > >> + result = SCAN_FAIL; > >> + __mod_lruvec_page_state(new_page, NR_FILE_THPS, -nr); > >> + filemap_nr_thps_dec(mapping); > >> + goto xa_locked; > >> + } > >> } > >> > >> if (nr_none) { > >> -- > >> 2.31.0.rc2.261.g7f71774620-goog > >> > > >
