[PATCH v5 2/2] mm,thp: Add experimental config option RO_EXEC_FILEMAP_HUGE_FAULT_THP

From: William Kucharski
Date: Mon Sep 02 2019 - 05:24:20 EST

Add filemap_huge_fault() to attempt to satisfy page
faults on memory-mapped read-only text pages using THP when possible.

Signed-off-by: William Kucharski <william.kucharski@xxxxxxxxxx>
---
include/linux/mm.h | 2 +
mm/Kconfig | 15 ++
mm/filemap.c | 398 +++++++++++++++++++++++++++++++++++++++++++--
mm/huge_memory.c | 3 +
mm/mmap.c | 39 ++++-
mm/rmap.c | 4 +-
mm/vmscan.c | 2 +-
7 files changed, 446 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0334ca97c584..2a5311721739 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2433,6 +2433,8 @@ extern void truncate_inode_pages_final(struct address_space *);

/* generic vm_area_ops exported for stackable file systems */
extern vm_fault_t filemap_fault(struct vm_fault *vmf);
+extern vm_fault_t filemap_huge_fault(struct vm_fault *vmf,
+ enum page_entry_size pe_size);
extern void filemap_map_pages(struct vm_fault *vmf,
pgoff_t start_pgoff, pgoff_t end_pgoff);
extern vm_fault_t filemap_page_mkwrite(struct vm_fault *vmf);
diff --git a/mm/Kconfig b/mm/Kconfig
index 56cec636a1fc..2debaded0e4d 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -736,4 +736,19 @@ config ARCH_HAS_PTE_SPECIAL
config ARCH_HAS_HUGEPD
bool

+config RO_EXEC_FILEMAP_HUGE_FAULT_THP
+ bool "read-only exec filemap_huge_fault THP support (EXPERIMENTAL)"
+ depends on TRANSPARENT_HUGE_PAGECACHE && SHMEM
+
+ help
+ Introduce filemap_huge_fault() to automatically map executable
+ read-only pages of mapped files of suitable size and alignment
+ using THP if possible.
+
+ This is marked experimental because it is a new feature and is
+ dependent upon filesystmes implementing readpages() in a way
+ that will recognize large THP pages and read file content to
+ them without polluting the pagecache with PAGESIZE pages due
+ to readahead.
+
endmenu
diff --git a/mm/filemap.c b/mm/filemap.c
index 38b46fc00855..5947d432a4e6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -199,13 +199,12 @@ static void unaccount_page_cache_page(struct address_space *mapping,
nr = hpage_nr_pages(page);

__mod_node_page_state(page_pgdat(page), NR_FILE_PAGES, -nr);
- if (PageSwapBacked(page)) {
+
+ if (PageSwapBacked(page))
__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);
- }
+
+ if (PageTransHuge(page))
+ __dec_node_page_state(page, NR_SHMEM_THPS);

/*
* At this point page must be either written or cleaned by
@@ -303,6 +302,9 @@ static void page_cache_delete_batch(struct address_space *mapping,
break;
if (xa_is_value(page))
continue;
+
+VM_BUG_ON_PAGE(xa_is_internal(page), page);
+
if (!tail_pages) {
/*
* Some page got inserted in our range? Skip it. We
@@ -315,6 +317,11 @@ static void page_cache_delete_batch(struct address_space *mapping,
continue;
}
WARN_ON_ONCE(!PageLocked(page));
+
+ /*
+ * If a THP is in the page cache, set the succeeding
+ * cache entries for the PMD-sized page to NULL.
+ */
if (PageTransHuge(page) && !PageHuge(page))
tail_pages = HPAGE_PMD_NR - 1;
page->mapping = NULL;
@@ -324,8 +331,6 @@ static void page_cache_delete_batch(struct address_space *mapping,
*/
i++;
} else {
- VM_BUG_ON_PAGE(page->index + HPAGE_PMD_NR - tail_pages
- != pvec->pages[i]->index, page);
tail_pages--;
}
xas_store(&xas, NULL);
@@ -881,7 +886,10 @@ static int __add_to_page_cache_locked(struct page *page,
mapping->nrpages++;

/* hugetlb pages do not participate in page cache accounting */
- if (!huge)
+ if (PageTransHuge(page) && !huge)
+ __mod_node_page_state(page_pgdat(page),
+ NR_FILE_PAGES, HPAGE_PMD_NR);
+ else
__inc_node_page_state(page, NR_FILE_PAGES);
unlock:
xas_unlock_irq(&xas);
@@ -1663,7 +1671,8 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
no_page:
if (!page && (fgp_flags & FGP_CREAT)) {
int err;
- if ((fgp_flags & FGP_WRITE) && mapping_cap_account_dirty(mapping))
+ if ((fgp_flags & FGP_WRITE) &&
+ mapping_cap_account_dirty(mapping))
gfp_mask |= __GFP_WRITE;
if (fgp_flags & FGP_NOFS)
gfp_mask &= ~__GFP_FS;
@@ -2643,6 +2652,372 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
}
EXPORT_SYMBOL(filemap_fault);

+#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
+/*
+ * There is a change coming to store only the head page of a compound page in
+ * the head cache.
+ *
+ * When that change is present in the kernel, remove this #define
+ */
+#define PAGE_CACHE_STORE_COMPOUND_TAIL_PAGES
+
+/*
+ * Check for an entry in the page cache which would conflict with the address
+ * range we wish to map using a THP or is otherwise unusable to map a large
+ * cached page.
+ *
+ * The routine will return true if a usable page is found in the page cache
+ * (and *pagep will be set to the address of the cached page), or if no
+ * cached page is found (and *pagep will be set to NULL).
+ */
+static bool
+filemap_huge_check_pagecache_usable(struct xa_state *xas,
+ struct page **pagep, pgoff_t hindex, pgoff_t hindex_max)
+{
+ struct page *page;
+
+ while (1) {
+ xas_set(xas, hindex);
+ page = xas_find(xas, hindex_max);
+
+ if (xas_retry(xas, page))
+ continue;
+
+ /*
+ * A found entry is unusable if:
+ * + the entry is an Xarray value, not a pointer
+ * + the entry is an internal Xarray node
+ * + the entry is not a compound page
+ * + the order of the compound page is < HPAGE_PMD_ORDER
+ * + the page index is not what we expect it to be
+ */
+ if (!page)
+ break;
+
+ if (xa_is_value(page) || xa_is_internal(page))
+ return false;
+
+#ifdef PAGE_CACHE_STORE_COMPOUND_TAIL_PAGES
+ if ((!PageCompound(page)) || (page != compound_head(page)))
+#else
+ if (!PageCompound(page))
+#endif
+ return false;
+
+ if (compound_order(page) < HPAGE_PMD_ORDER)
+ return false;
+
+ if (page->index != hindex)
+ return false;
+
+ break;
+ }
+
+ *pagep = page;
+ return true;
+}
+
+/**
+ * filemap_huge_fault - read in file data for page fault handling to THP
+ * @vmf: struct vm_fault containing details of the fault
+ * @pe_size: large page size to map, currently this must be PE_SIZE_PMD
+ *
+ * filemap_huge_fault() is invoked via the vma operations vector for a
+ * mapped memory region to read in file data to a transparent huge page during
+ * a page fault.
+ *
+ * If for any reason we can't allocate a THP, map it or add it to the page
+ * cache, VM_FAULT_FALLBACK will be returned which will cause the fault
+ * handler to try mapping the page using a PAGESIZE page, usually via
+ * filemap_fault() if so speicifed in the vma operations vector.
+ *
+ * Returns either VM_FAULT_FALLBACK or the result of calling allcc_set_pte()
+ * to map the new THP.
+ *
+ * NOTE: This routine depends upon the file system's readpage routine as
+ * specified in the address space operations vector to recognize when it
+ * is being passed a large page and to read the approprate amount of data
+ * in full and without polluting the page cache for the large page itself
+ * with PAGESIZE pages to perform a buffered read or to pollute what
+ * would be the page cache space for any succeeding pages with PAGESIZE
+ * pages due to readahead.
+ *
+ * It is VITAL that this routine not be enabled without such filesystem
+ * support. As there is no way to determine how many bytes were read by
+ * the readpage() operation, if only a PAGESIZE page is read, this routine
+ * will map the THP containing only the first PAGESIZE bytes of file data
+ * to satisfy the fault, which is never the result desired.
+ */
+vm_fault_t filemap_huge_fault(struct vm_fault *vmf,
+ enum page_entry_size pe_size)
+{
+ struct file *filp = vmf->vma->vm_file;
+ struct address_space *mapping = filp->f_mapping;
+ struct vm_area_struct *vma = vmf->vma;
+
+ unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+ pgoff_t hindex = round_down(vmf->pgoff, HPAGE_PMD_NR);
+ pgoff_t hindex_max = hindex + HPAGE_PMD_NR - 1;
+
+ struct page *cached_page, *hugepage;
+ struct page *new_page = NULL;
+
+ vm_fault_t ret = VM_FAULT_FALLBACK;
+ unsigned long nr;
+
+ int error;
+ bool retry_lookup = true;
+
+ XA_STATE_ORDER(xas, &mapping->i_pages, hindex, HPAGE_PMD_ORDER);
+
+ /*
+ * Return VM_FAULT_FALLBACK if:
+ *
+ * + pe_size != PE_SIZE_PMD
+ * + FAULT_FLAG_WRITE is set in vmf->flags
+ * + vma isn't aligned to allow a PMD mapping
+ * + PMD would extend beyond the end of the vma
+ */
+ if (pe_size != PE_SIZE_PMD || (vmf->flags & FAULT_FLAG_WRITE) ||
+ (haddr < vma->vm_start ||
+ ((haddr + HPAGE_PMD_SIZE) > vma->vm_end)))
+ return ret;
+
+retry_lookup:
+ rcu_read_lock();
+
+ if (!filemap_huge_check_pagecache_usable(&xas, &cached_page, hindex,
+ hindex_max)) {
+ /* found a conflicting entry in the page cache, so fallback */
+ rcu_read_unlock();
+ return ret;
+ } else if (cached_page) {
+ /* found a valid cached page, so map it */
+ rcu_read_unlock();
+ lock_page(cached_page);
+
+ /* was the cached page truncated while waiting for the lock? */
+ if (unlikely(cached_page->mapping != mapping)) {
+ unlock_page(cached_page);
+
+ /* retry once */
+ if (retry_lookup) {
+ retry_lookup = false;
+ goto retry_lookup;
+ }
+
+ return ret;
+ }
+
+ if (unlikely(!PageUptodate(cached_page))) {
+ unlock_page(cached_page);
+ return ret;
+ }
+
+ VM_BUG_ON_PAGE(cached_page->index != hindex, cached_page);
+
+ hugepage = cached_page;
+ goto map_huge;
+ }
+
+ rcu_read_unlock();
+
+ /* allocate huge THP page in VMA */
+ new_page = __page_cache_alloc(vmf->gfp_mask | __GFP_COMP |
+ __GFP_NOWARN | __GFP_NORETRY, HPAGE_PMD_ORDER);
+
+ if (unlikely(!new_page))
+ return ret;
+
+ do {
+ xas_lock_irq(&xas);
+ xas_set(&xas, hindex);
+ xas_create_range(&xas);
+
+ if (!(xas_error(&xas)))
+ break;
+
+ xas_unlock_irq(&xas);
+
+ if (!xas_nomem(&xas, GFP_KERNEL)) {
+ /* error creating range, so free THP and fallback */
+ if (new_page)
+ put_page(new_page);
+
+ return ret;
+ }
+ } while (1);
+
+ /* i_pages is locked here */
+
+ /*
+ * Double check that an entry did not sneak into the page cache while
+ * creating Xarray entries for the new page.
+ */
+ if (!filemap_huge_check_pagecache_usable(&xas, &cached_page, hindex,
+ hindex_max)) {
+ /*
+ * An unusable entry was found, so delete the newly allocated
+ * page and fallback.
+ */
+ put_page(new_page);
+ xas_unlock_irq(&xas);
+ return ret;
+ } else if (cached_page) {
+ /*
+ * A valid large page was found in the page cache, so free the
+ * newly allocated page and map the cached page instead.
+ */
+ put_page(new_page);
+ new_page = NULL;
+ xas_unlock_irq(&xas);
+
+ lock_page(cached_page);
+
+ /* was the cached page truncated while waiting for the lock? */
+ if (unlikely(cached_page->mapping != mapping)) {
+ unlock_page(cached_page);
+
+ /* retry once */
+ if (retry_lookup) {
+ retry_lookup = false;
+ goto retry_lookup;
+ }
+
+ return ret;
+ }
+
+ if (unlikely(!PageUptodate(cached_page))) {
+ unlock_page(cached_page);
+ return ret;
+ }
+
+ VM_BUG_ON_PAGE(cached_page->index != hindex, cached_page);
+
+ hugepage = cached_page;
+ goto map_huge;
+ }
+
+ prep_transhuge_page(new_page);
+ new_page->mapping = mapping;
+ new_page->index = hindex;
+ __SetPageLocked(new_page);
+
+ count_vm_event(THP_FILE_ALLOC);
+ xas_set(&xas, hindex);
+
+ for (nr = 0; nr < HPAGE_PMD_NR; nr++) {
+#ifdef PAGE_CACHE_STORE_COMPOUND_TAIL_PAGES
+ /*
+ * Store pointers to both head and tail pages of a compound
+ * page in the page cache.
+ */
+ xas_store(&xas, new_page + nr);
+#else
+ /*
+ * All entries for a compound page in the page cache should
+ * point to the head page.
+ */
+ xas_store(&xas, new_page);
+#endif
+ xas_next(&xas);
+ }
+
+ mapping->nrpages += HPAGE_PMD_NR;
+ xas_unlock_irq(&xas);
+
+ /*
+ * The readpage() operation below is expected to fill the large
+ * page with data without polluting the page cache with
+ * PAGESIZE entries due to a buffered read and/or readahead().
+ *
+ * A filesystem's vm_operations_struct huge_fault field should
+ * never point to this routine without such a capability, and
+ * without it a call to this routine would eventually just
+ * fall through to the normal fault op anyway.
+ */
+ error = mapping->a_ops->readpage(vmf->vma->vm_file, new_page);
+
+ if (unlikely(error)) {
+ ret = VM_FAULT_SIGBUS;
+ goto delete_hugepage_from_page_cache;
+ }
+
+ if (wait_on_page_locked_killable(new_page)) {
+ ret = VM_FAULT_SIGSEGV;
+ goto delete_hugepage_from_page_cache;
+ }
+
+ if (!PageUptodate(new_page)) {
+ /* EIO */
+ ret = VM_FAULT_SIGBUS;
+ goto delete_hugepage_from_page_cache;
+ }
+
+ lock_page(new_page);
+
+ /* did the page get truncated while waiting for the lock? */
+ if (unlikely(new_page->mapping != mapping)) {
+ unlock_page(new_page);
+ goto delete_hugepage_from_page_cache;
+ }
+
+ __inc_node_page_state(new_page, NR_SHMEM_THPS);
+ __mod_node_page_state(page_pgdat(new_page),
+ NR_FILE_PAGES, HPAGE_PMD_NR);
+ __mod_node_page_state(page_pgdat(new_page),
+ NR_SHMEM, HPAGE_PMD_NR);
+
+ hugepage = new_page;
+
+map_huge:
+ /* map hugepage at the PMD level */
+
+ ret = alloc_set_pte(vmf, vmf->memcg, hugepage);
+
+ VM_BUG_ON_PAGE((!(pmd_trans_huge(*vmf->pmd))), hugepage);
+ VM_BUG_ON_PAGE(!(PageTransHuge(hugepage)), hugepage);
+
+ if (likely(!(ret & VM_FAULT_ERROR))) {
+ vmf->address = haddr;
+ vmf->page = hugepage;
+
+ page_ref_add(hugepage, HPAGE_PMD_NR);
+ count_vm_event(THP_FILE_MAPPED);
+ } else {
+ if (new_page) {
+ __mod_node_page_state(page_pgdat(new_page),
+ NR_FILE_PAGES, -HPAGE_PMD_NR);
+ __mod_node_page_state(page_pgdat(new_page),
+ NR_SHMEM, -HPAGE_PMD_NR);
+ __dec_node_page_state(new_page, NR_SHMEM_THPS);
+
+delete_hugepage_from_page_cache:
+ xas_lock_irq(&xas);
+ xas_set(&xas, hindex);
+
+ for (nr = 0; nr < HPAGE_PMD_NR; nr++) {
+ xas_store(&xas, NULL);
+ xas_next(&xas);
+ }
+
+ new_page->mapping = NULL;
+ xas_unlock_irq(&xas);
+
+ mapping->nrpages -= HPAGE_PMD_NR;
+ unlock_page(new_page);
+ page_ref_dec(new_page); /* decrement page coche ref */
+ put_page(new_page); /* done with page */
+ return ret;
+ }
+ }
+
+ unlock_page(hugepage);
+ return ret;
+}
+EXPORT_SYMBOL(filemap_huge_fault);
+#endif
+
void filemap_map_pages(struct vm_fault *vmf,
pgoff_t start_pgoff, pgoff_t end_pgoff)
{
@@ -2925,7 +3300,8 @@ struct page *read_cache_page(struct address_space *mapping,
EXPORT_SYMBOL(read_cache_page);

/**
- * read_cache_page_gfp - read into page cache, using specified page allocation flags.
+ * read_cache_page_gfp - read into page cache, using specified page allocation
+ * flags.
* @mapping: the page's address_space
* @index: the page index
* @gfp: the page allocator flags to use if allocating
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de1f15969e27..ea3dbb6fa538 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -544,8 +544,11 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,

if (addr)
goto out;
+
+#ifndef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
if (!IS_DAX(filp->f_mapping->host) || !IS_ENABLED(CONFIG_FS_DAX_PMD))
goto out;
+#endif

addr = __thp_get_unmapped_area(filp, len, off, flags, PMD_SIZE);
if (addr)
diff --git a/mm/mmap.c b/mm/mmap.c
index 7e8c3e8ae75f..d8b3bce71075 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1391,6 +1391,8 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
struct mm_struct *mm = current->mm;
int pkey = 0;

+ unsigned long vm_maywrite = VM_MAYWRITE;
+
*populate = 0;

if (!len)
@@ -1426,10 +1428,41 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;

- /* Obtain the address to map to. we verify (or select) it and ensure
+ /*
+ * Obtain the address to map to. we verify (or select) it and ensure
* that it represents a valid section of the address space.
*/
- addr = get_unmapped_area(file, addr, len, pgoff, flags);
+
+#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
+ /*
+ * If THP is enabled, it's a read-only executable that is
+ * MAP_PRIVATE mapped, the length is larger than a PMD page
+ * and either it's not a MAP_FIXED mapping or the passed address is
+ * properly aligned for a PMD page, attempt to get an appropriate
+ * address at which to map a PMD-sized THP page, otherwise call the
+ * normal routine.
+ */
+ if ((prot & PROT_READ) && (prot & PROT_EXEC) &&
+ (!(prot & PROT_WRITE)) && (flags & MAP_PRIVATE) &&
+ (!(flags & MAP_FIXED)) && len >= HPAGE_PMD_SIZE) {
+ addr = thp_get_unmapped_area(file, addr, len, pgoff, flags);
+
+ if (addr && (!(addr & ~HPAGE_PMD_MASK))) {
+ /*
+ * If we got a suitable THP mapping address, shut off
+ * VM_MAYWRITE for the region, since it's never what
+ * we would want.
+ */
+ vm_maywrite = 0;
+ } else
+ addr = get_unmapped_area(file, addr, len, pgoff, flags);
+ } else {
+#endif
+ addr = get_unmapped_area(file, addr, len, pgoff, flags);
+#ifdef CONFIG_RO_EXEC_FILEMAP_HUGE_FAULT_THP
+ }
+#endif
+
if (offset_in_page(addr))
return addr;

@@ -1451,7 +1484,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
* of the memory object, so we don't do any here.
*/
vm_flags |= calc_vm_prot_bits(prot, pkey) | calc_vm_flag_bits(flags) |
- mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
+ mm->def_flags | VM_MAYREAD | vm_maywrite | VM_MAYEXEC;

if (flags & MAP_LOCKED)
if (!can_do_mlock())
diff --git a/mm/rmap.c b/mm/rmap.c
index 003377e24232..aacc6e330329 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1192,7 +1192,7 @@ 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);
} else {
if (PageTransCompound(page) && page_mapping(page)) {
@@ -1232,7 +1232,7 @@ 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);
} else {
if (!atomic_add_negative(-1, &page->_mapcount))
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a6c5d0b28321..47a19c59c9a2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -930,7 +930,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
* Note that if SetPageDirty is always performed via set_page_dirty,
* and thus under the i_pages lock, then this ordering is not required.
*/
- if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
+ if (unlikely(PageTransHuge(page)))
refcount = 1 + HPAGE_PMD_NR;
else
refcount = 2;
--
2.21.0