[Qemu-devel] [PATCH 10/17] mm: rmap preparation for remap_anon_pages

[Andrea Arcangeli]

remap_anon_pages (unlike remap_file_pages) tries to be non intrusive
in the rmap code.

As far as the rmap code is concerned, rmap_anon_pages only alters the
page->mapping and page->index. It does it while holding the page
lock. However there are a few places that in presence of anon pages
are allowed to do rmap walks without the page lock (split_huge_page
and page_referenced_anon). Those places that are doing rmap walks
without taking the page lock first, must be updated to re-check that
the page->mapping didn't change after they obtained the anon_vma
lock. remap_anon_pages takes the anon_vma lock for writing before
altering the page->mapping, so if the page->mapping is still the same
after obtaining the anon_vma lock (without the page lock), the rmap
walks can go ahead safely (and remap_anon_pages will wait them to
complete before proceeding).

remap_anon_pages serializes against itself with the page lock.

All other places taking the anon_vma lock while holding the mmap_sem
for writing, don't need to check if the page->mapping has changed
after taking the anon_vma lock, regardless of the page lock, because
remap_anon_pages holds the mmap_sem for reading.

Overall this looks a fairly small change to the rmap code, notably
less intrusive than the nonlinear vmas created by remap_file_pages.

There's one constraint enforced to allow this simplification: the
source pages passed to remap_anon_pages must be mapped only in one
vma, but this is not a limitation when used to handle userland page
faults with MADV_USERFAULT. The source addresses passed to
remap_anon_pages should be set as VM_DONTCOPY with MADV_DONTFORK to
avoid any risk of the mapcount of the pages increasing, if fork runs
in parallel in another thread, before or while remap_anon_pages runs.

Signed-off-by: Andrea Arcangeli <address@hidden>
---
 mm/huge_memory.c | 24 ++++++++++++++++++++----
 mm/rmap.c        |  9 +++++++++
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b402d60..4277ed7 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1921,6 +1921,7 @@ int split_huge_page_to_list(struct page *page, struct 
list_head *list)
 {
        struct anon_vma *anon_vma;
        int ret = 1;
+       struct address_space *mapping;
 
        BUG_ON(is_huge_zero_page(page));
        BUG_ON(!PageAnon(page));
@@ -1932,10 +1933,24 @@ int split_huge_page_to_list(struct page *page, struct 
list_head *list)
         * page_lock_anon_vma_read except the write lock is taken to serialise
         * against parallel split or collapse operations.
         */
-       anon_vma = page_get_anon_vma(page);
-       if (!anon_vma)
-               goto out;
-       anon_vma_lock_write(anon_vma);
+       for (;;) {
+               mapping = ACCESS_ONCE(page->mapping);
+               anon_vma = page_get_anon_vma(page);
+               if (!anon_vma)
+                       goto out;
+               anon_vma_lock_write(anon_vma);
+               /*
+                * We don't hold the page lock here so
+                * remap_anon_pages_huge_pmd can change the anon_vma
+                * from under us until we obtain the anon_vma
+                * lock. Verify that we obtained the anon_vma lock
+                * before remap_anon_pages did.
+                */
+               if (likely(mapping == ACCESS_ONCE(page->mapping)))
+                       break;
+               anon_vma_unlock_write(anon_vma);
+               put_anon_vma(anon_vma);
+       }
 
        ret = 0;
        if (!PageCompound(page))
@@ -2460,6 +2475,7 @@ static void collapse_huge_page(struct mm_struct *mm,
         * Prevent all access to pagetables with the exception of
         * gup_fast later hanlded by the ptep_clear_flush and the VM
         * handled by the anon_vma lock + PG_lock.
+        * remap_anon_pages is prevented to race as well by the mmap_sem.
         */
        down_write(&mm->mmap_sem);
        if (unlikely(khugepaged_test_exit(mm)))
diff --git a/mm/rmap.c b/mm/rmap.c
index 3e8491c..6d875eb 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -450,6 +450,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
        struct anon_vma *root_anon_vma;
        unsigned long anon_mapping;
 
+repeat:
        rcu_read_lock();
        anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
@@ -488,6 +489,14 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
        rcu_read_unlock();
        anon_vma_lock_read(anon_vma);
 
+       /* check if remap_anon_pages changed the anon_vma */
+       if (unlikely((unsigned long) ACCESS_ONCE(page->mapping) != 
anon_mapping)) {
+               anon_vma_unlock_read(anon_vma);
+               put_anon_vma(anon_vma);
+               anon_vma = NULL;
+               goto repeat;
+       }
+
        if (atomic_dec_and_test(&anon_vma->refcount)) {
                /*
                 * Oops, we held the last refcount, release the lock