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Date: Thu, 22 Apr 2021 00:08:58 +0000
From: Song Liu <songliubraving@...com>
To: Collin Fijalkovich <cfijalkovich@...gle.com>
CC: Suren Baghdasaryan <surenb@...gle.com>,
"hridya@...gle.com" <hridya@...gle.com>,
"kaleshsingh@...gle.com" <kaleshsingh@...gle.com>,
"hughd@...gle.com" <hughd@...gle.com>,
"timmurray@...gle.com" <timmurray@...gle.com>,
"william.kucharski@...cle.com" <william.kucharski@...cle.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"willy@...radead.org" <willy@...radead.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm, thp: Relax the VM_DENYWRITE constraint on
file-backed THPs
> On Apr 5, 2021, at 5:09 PM, Collin Fijalkovich <cfijalkovich@...gle.com> wrote:
>
> Transparent huge pages are supported for read-only non-shmem files,
> 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.
>
> Comparison of the performance characteristics of 4KB and 2MB-backed
> libraries follows; the Android dex2oat tool was used to AOT compile an
> example application on a single ARM core.
>
> 4KB Pages:
> ==========
>
> count event_name # count / runtime
> 598,995,035,942 cpu-cycles # 1.800861 GHz
> 81,195,620,851 raw-stall-frontend # 244.112 M/sec
> 347,754,466,597 iTLB-loads # 1.046 G/sec
> 2,970,248,900 iTLB-load-misses # 0.854122% miss rate
>
> Total test time: 332.854998 seconds.
>
> 2MB Pages:
> ==========
>
> count event_name # count / runtime
> 592,872,663,047 cpu-cycles # 1.800358 GHz
> 76,485,624,143 raw-stall-frontend # 232.261 M/sec
> 350,478,413,710 iTLB-loads # 1.064 G/sec
> 803,233,322 iTLB-load-misses # 0.229182% miss rate
>
> Total test time: 329.826087 seconds
>
> A check of /proc/$(pidof dex2oat64)/smaps shows THPs in use:
>
> /apex/com.android.art/lib64/libart.so
> FilePmdMapped: 4096 kB
>
> /apex/com.android.art/lib64/libart-compiler.so
> FilePmdMapped: 2048 kB
>
> Signed-off-by: Collin Fijalkovich <cfijalkovich@...gle.com>
Acked-by: Song Liu <song@...nel.org>
> ---
> Changes v1 -> v2:
> * commit message 'non-shmem filesystems' -> 'non-shmem files'
> * Add performance testing data to commit message
>
> 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) {
> --
> 2.31.0.208.g409f899ff0-goog
>
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