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
<cfijalkov...@google.com> 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 <cfijalkov...@google.com>
> ---
> 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
>
