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Subject[PATCH v8 5/6] mm,thp: add read-only THP support for (non-shmem) FS
Date
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. sys_mmap() ignores VM_DENYWRITE requests
(see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is
execve(). This requirement limits non-shmem THP to text sections.

The next patch will handle writes, which would only happen when the all
the vmas with VM_DENYWRITE are 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 | 94 +++++++++++++++++++++++++++++++++++++++++--------
mm/rmap.c | 12 ++++---
4 files changed, 100 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..acbbbeaa083c 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 +1322,9 @@ 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);

+ 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 +1356,8 @@ 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 +1372,7 @@ 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 +1393,7 @@ 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 +1401,29 @@ 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);
+ /* drain pagevecs to help isolate_lru_page() */
+ lru_add_drain();
+ page = find_lock_page(mapping, index);
+ if (unlikely(page == NULL)) {
+ result = SCAN_FAIL;
+ goto xa_unlocked;
+ }
+ } else if (!PageUptodate(page)) {
+ VM_BUG_ON(is_shmem);
+ xas_unlock_irq(&xas);
+ wait_on_page_locked(page);
+ if (!trylock_page(page)) {
+ result = SCAN_PAGE_LOCK;
+ goto xa_unlocked;
+ }
+ get_page(page);
+ } else if (!is_shmem && PageDirty(page)) {
+ result = SCAN_FAIL;
+ goto xa_locked;
} else if (trylock_page(page)) {
get_page(page);
xas_unlock_irq(&xas);
@@ -1426,6 +1458,12 @@ static void collapse_file(struct mm_struct *mm,
goto out_unlock;
}

+ if (page_has_private(page) &&
+ !try_to_release_page(page, GFP_KERNEL)) {
+ result = SCAN_PAGE_HAS_PRIVATE;
+ break;
+ }
+
if (page_mapped(page))
unmap_mapping_pages(mapping, index, 1, false);

@@ -1463,12 +1501,18 @@ static void collapse_file(struct mm_struct *mm,
goto xa_unlocked;
}

- __inc_node_page_state(new_page, NR_SHMEM_THPS);
+ if (is_shmem)
+ __inc_node_page_state(new_page, NR_SHMEM_THPS);
+ else
+ __inc_node_page_state(new_page, NR_FILE_THPS);
+
if (nr_none) {
struct zone *zone = page_zone(new_page);

__mod_node_page_state(zone->zone_pgdat, NR_FILE_PAGES, nr_none);
- __mod_node_page_state(zone->zone_pgdat, NR_SHMEM, nr_none);
+ if (is_shmem)
+ __mod_node_page_state(zone->zone_pgdat,
+ NR_SHMEM, nr_none);
}

xa_locked:
@@ -1506,10 +1550,15 @@ static void collapse_file(struct mm_struct *mm,

SetPageUptodate(new_page);
page_ref_add(new_page, HPAGE_PMD_NR - 1);
- set_page_dirty(new_page);
mem_cgroup_commit_charge(new_page, memcg, false, true);
+
+ if (is_shmem) {
+ set_page_dirty(new_page);
+ lru_cache_add_anon(new_page);
+ } else {
+ lru_cache_add_file(new_page);
+ }
count_memcg_events(memcg, THP_COLLAPSE_ALLOC, 1);
- lru_cache_add_anon(new_page);

/*
* Remove pte page tables, so we can re-fault the page as huge.
@@ -1524,7 +1573,9 @@ static void collapse_file(struct mm_struct *mm,
/* Something went wrong: roll back page cache changes */
xas_lock_irq(&xas);
mapping->nrpages -= nr_none;
- shmem_uncharge(mapping->host, nr_none);
+
+ if (is_shmem)
+ shmem_uncharge(mapping->host, nr_none);

xas_set(&xas, start);
xas_for_each(&xas, page, end - 1) {
@@ -1607,6 +1658,17 @@ static void khugepaged_scan_file(struct mm_struct *mm,
break;
}

+ if (page_has_private(page) && trylock_page(page)) {
+ int ret;
+
+ ret = try_to_release_page(page, GFP_KERNEL);
+ unlock_page(page);
+ if (!ret) {
+ result = SCAN_PAGE_HAS_PRIVATE;
+ break;
+ }
+ }
+
if (page_count(page) != 1 + page_mapcount(page)) {
result = SCAN_PAGE_COUNT;
break;
@@ -1713,11 +1775,13 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
VM_BUG_ON(khugepaged_scan.address < hstart ||
khugepaged_scan.address + HPAGE_PMD_SIZE >
hend);
- if (shmem_file(vma->vm_file)) {
+ if (vma->vm_file) {
struct file *file;
pgoff_t pgoff = linear_page_index(vma,
khugepaged_scan.address);
- if (!shmem_huge_enabled(vma))
+
+ if (shmem_file(vma->vm_file)
+ && !shmem_huge_enabled(vma))
goto skip;
file = get_file(vma->vm_file);
up_read(&mm->mmap_sem);
diff --git a/mm/rmap.c b/mm/rmap.c
index e5dfe2ae6b0d..87cfa2c19eda 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1192,8 +1192,10 @@ void page_add_file_rmap(struct page *page, bool compound)
}
if (!atomic_inc_and_test(compound_mapcount_ptr(page)))
goto out;
- VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
- __inc_node_page_state(page, NR_SHMEM_PMDMAPPED);
+ if (PageSwapBacked(page))
+ __inc_node_page_state(page, NR_SHMEM_PMDMAPPED);
+ else
+ __inc_node_page_state(page, NR_FILE_PMDMAPPED);
} else {
if (PageTransCompound(page) && page_mapping(page)) {
VM_WARN_ON_ONCE(!PageLocked(page));
@@ -1232,8 +1234,10 @@ static void page_remove_file_rmap(struct page *page, bool compound)
}
if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
goto out;
- VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
- __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
+ if (PageSwapBacked(page))
+ __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
+ else
+ __dec_node_page_state(page, NR_FILE_PMDMAPPED);
} else {
if (!atomic_add_negative(-1, &page->_mapcount))
goto out;
--
2.17.1
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