From: Collin Fijalkovich <cfijalkovich@xxxxxxxxxx> Subject: mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs 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 Link: https://lkml.kernel.org/r/20210406000930.3455850-1-cfijalkovich@xxxxxxxxxx Signed-off-by: Collin Fijalkovich <cfijalkovich@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx> Acked-by: Song Liu <song@xxxxxxxxxx> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx> Cc: Hridya Valsaraju <hridya@xxxxxxxxxx> Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx> Cc: Tim Murray <timmurray@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/open.c | 13 +++++++++++-- mm/khugepaged.c | 16 +++++++++++++++- 2 files changed, 26 insertions(+), 3 deletions(-) --- a/fs/open.c~mm-thp-relax-the-vm_denywrite-constraint-on-file-backed-thps +++ a/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; --- a/mm/khugepaged.c~mm-thp-relax-the-vm_denywrite-constraint-on-file-backed-thps +++ a/mm/khugepaged.c @@ -457,7 +457,8 @@ static bool hugepage_vma_check(struct vm /* 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); } @@ -1862,6 +1863,19 @@ out_unlock: 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) { _