Re: [PATCH] mm, thp: Relax the VM_DENYWRITE constraint on file-backed THPs

[William Kucharski]

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
>> 
>