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SubjectRe: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS
On Sat, Jun 22, 2019 at 10:47:48PM -0700, Song Liu wrote:
> This patch is (hopefully) the first step to enable THP for non-shmem
> filesystems.
>
> This patch enables an application to put part of its text sections to THP
> via madvise, for example:
>
> madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
>
> We tried to reuse the logic for THP on tmpfs.
>
> Currently, write is not supported for non-shmem THP. khugepaged will only
> process vma with VM_DENYWRITE. The next patch will handle writes, which
> would only happen when the vma with VM_DENYWRITE is unmapped.
>
> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
> feature.
>
> Acked-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
> mm/Kconfig | 11 ++++++
> mm/filemap.c | 4 +--
> mm/khugepaged.c | 90 ++++++++++++++++++++++++++++++++++++++++---------
> mm/rmap.c | 12 ++++---
> 4 files changed, 96 insertions(+), 21 deletions(-)
>
> diff --git a/mm/Kconfig b/mm/Kconfig
> index f0c76ba47695..0a8fd589406d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -762,6 +762,17 @@ config GUP_BENCHMARK
>
> See tools/testing/selftests/vm/gup_benchmark.c
>
> +config READ_ONLY_THP_FOR_FS
> + bool "Read-only THP for filesystems (EXPERIMENTAL)"
> + depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
> +
> + help
> + Allow khugepaged to put read-only file-backed pages in THP.
> +
> + This is marked experimental because it is a new feature. Write
> + support of file THPs will be developed in the next few release
> + cycles.
> +
> config ARCH_HAS_PTE_SPECIAL
> bool
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 5f072a113535..e79ceccdc6df 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -203,8 +203,8 @@ static void unaccount_page_cache_page(struct address_space *mapping,
> __mod_node_page_state(page_pgdat(page), NR_SHMEM, -nr);
> if (PageTransHuge(page))
> __dec_node_page_state(page, NR_SHMEM_THPS);
> - } else {
> - VM_BUG_ON_PAGE(PageTransHuge(page), page);
> + } else if (PageTransHuge(page)) {
> + __dec_node_page_state(page, NR_FILE_THPS);
> }
>
> /*
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 158cad542627..090127e4e185 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -48,6 +48,7 @@ enum scan_result {
> SCAN_CGROUP_CHARGE_FAIL,
> SCAN_EXCEED_SWAP_PTE,
> SCAN_TRUNCATED,
> + SCAN_PAGE_HAS_PRIVATE,
> };
>
> #define CREATE_TRACE_POINTS
> @@ -404,7 +405,11 @@ static bool hugepage_vma_check(struct vm_area_struct *vma,
> (vm_flags & VM_NOHUGEPAGE) ||
> test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> return false;
> - if (shmem_file(vma->vm_file)) {
> +
> + if (shmem_file(vma->vm_file) ||
> + (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> + vma->vm_file &&
> + (vm_flags & VM_DENYWRITE))) {
> if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
> return false;
> return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
> @@ -456,8 +461,9 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
> unsigned long hstart, hend;
>
> /*
> - * khugepaged does not yet work on non-shmem files or special
> - * mappings. And file-private shmem THP is not supported.
> + * khugepaged only supports read-only files for non-shmem files.
> + * khugepaged does not yet work on special mappings. And
> + * file-private shmem THP is not supported.
> */
> if (!hugepage_vma_check(vma, vm_flags))
> return 0;
> @@ -1287,12 +1293,12 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> }
>
> /**
> - * collapse_file - collapse small tmpfs/shmem pages into huge one.
> + * collapse_file - collapse filemap/tmpfs/shmem pages into huge one.
> *
> * Basic scheme is simple, details are more complex:
> * - allocate and lock a new huge page;
> * - scan page cache replacing old pages with the new one
> - * + swap in pages if necessary;
> + * + swap/gup in pages if necessary;
> * + fill in gaps;
> * + keep old pages around in case rollback is required;
> * - if replacing succeeds:
> @@ -1316,7 +1322,11 @@ static void collapse_file(struct mm_struct *mm,
> LIST_HEAD(pagelist);
> XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> int nr_none = 0, result = SCAN_SUCCEED;
> + bool is_shmem = shmem_file(file);
>
> +#ifndef CONFIG_READ_ONLY_THP_FOR_FS
> + VM_BUG_ON(!is_shmem);
> +#endif

VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);

> VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
>
> /* Only allocate from the target node */
> @@ -1348,7 +1358,8 @@ static void collapse_file(struct mm_struct *mm,
> } while (1);
>
> __SetPageLocked(new_page);
> - __SetPageSwapBacked(new_page);
> + if (is_shmem)
> + __SetPageSwapBacked(new_page);
> new_page->index = start;
> new_page->mapping = mapping;
>
> @@ -1363,7 +1374,7 @@ static void collapse_file(struct mm_struct *mm,
> struct page *page = xas_next(&xas);
>
> VM_BUG_ON(index != xas.xa_index);
> - if (!page) {
> + if (is_shmem && !page) {
> /*
> * Stop if extent has been truncated or hole-punched,
> * and is now completely empty.
> @@ -1384,7 +1395,7 @@ static void collapse_file(struct mm_struct *mm,
> continue;
> }
>
> - if (xa_is_value(page) || !PageUptodate(page)) {
> + if (is_shmem && (xa_is_value(page) || !PageUptodate(page))) {
> xas_unlock_irq(&xas);
> /* swap in or instantiate fallocated page */
> if (shmem_getpage(mapping->host, index, &page,
> @@ -1392,6 +1403,23 @@ static void collapse_file(struct mm_struct *mm,
> result = SCAN_FAIL;
> goto xa_unlocked;
> }
> + } else if (!page || xa_is_value(page)) {
> + xas_unlock_irq(&xas);
> + page_cache_sync_readahead(mapping, &file->f_ra, file,
> + index, PAGE_SIZE);
> + lru_add_drain();

Why?

> + page = find_lock_page(mapping, index);
> + if (unlikely(page == NULL)) {
> + result = SCAN_FAIL;
> + goto xa_unlocked;
> + }
> + } else if (!PageUptodate(page)) {

Maybe we should try wait_on_page_locked() here before give up?

> + VM_BUG_ON(is_shmem);
> + result = SCAN_FAIL;
> + goto xa_locked;
> + } else if (!is_shmem && PageDirty(page)) {
> + result = SCAN_FAIL;
> + goto xa_locked;
> } else if (trylock_page(page)) {
> get_page(page);
> xas_unlock_irq(&xas);
--
Kirill A. Shutemov

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