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futex: Use a folio instead of a page

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The futex code already handles compound pages correctly, but using a folio
tells the compiler that there is already a reference to the head page and
it doesn't need to call compound_head() again.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230821142207.2537124-1-willy@infradead.org
This commit is contained in:
Matthew Wilcox (Oracle) 2023-08-21 15:22:07 +01:00 committed by Thomas Gleixner
parent 85be6d8424
commit e35a6cf1cc
1 changed files with 33 additions and 34 deletions
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67
kernel/futex/core.c
View file

@ -222,7 +222,8 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
struct page *page, *tail;
struct page *page;
struct folio *folio;
struct address_space *mapping;
int err, ro = 0;
@ -273,54 +274,52 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
err = 0;
/*
* The treatment of mapping from this point on is critical. The page
* lock protects many things but in this context the page lock
* The treatment of mapping from this point on is critical. The folio
* lock protects many things but in this context the folio lock
* stabilizes mapping, prevents inode freeing in the shared
* file-backed region case and guards against movement to swap cache.
*
* Strictly speaking the page lock is not needed in all cases being
* considered here and page lock forces unnecessarily serialization
* Strictly speaking the folio lock is not needed in all cases being
* considered here and folio lock forces unnecessarily serialization.
* From this point on, mapping will be re-verified if necessary and
* page lock will be acquired only if it is unavoidable
* folio lock will be acquired only if it is unavoidable
*
* Mapping checks require the head page for any compound page so the
* head page and mapping is looked up now. For anonymous pages, it
* does not matter if the page splits in the future as the key is
* based on the address. For filesystem-backed pages, the tail is
* required as the index of the page determines the key. For
* base pages, there is no tail page and tail == page.
* Mapping checks require the folio so it is looked up now. For
* anonymous pages, it does not matter if the folio is split
* in the future as the key is based on the address. For
* filesystem-backed pages, the precise page is required as the
* index of the page determines the key.
*/
tail = page;
page = compound_head(page);
mapping = READ_ONCE(page->mapping);
folio = page_folio(page);
mapping = READ_ONCE(folio->mapping);
/*
* If page->mapping is NULL, then it cannot be a PageAnon
* If folio->mapping is NULL, then it cannot be an anonymous
* page; but it might be the ZERO_PAGE or in the gate area or
* in a special mapping (all cases which we are happy to fail);
* or it may have been a good file page when get_user_pages_fast
* found it, but truncated or holepunched or subjected to
* invalidate_complete_page2 before we got the page lock (also
* invalidate_complete_page2 before we got the folio lock (also
* cases which we are happy to fail). And we hold a reference,
* so refcount care in invalidate_inode_page's remove_mapping
* prevents drop_caches from setting mapping to NULL beneath us.
*
* The case we do have to guard against is when memory pressure made
* shmem_writepage move it from filecache to swapcache beneath us:
* an unlikely race, but we do need to retry for page->mapping.
* an unlikely race, but we do need to retry for folio->mapping.
*/
if (unlikely(!mapping)) {
int shmem_swizzled;
/*
* Page lock is required to identify which special case above
* applies. If this is really a shmem page then the page lock
* Folio lock is required to identify which special case above
* applies. If this is really a shmem page then the folio lock
* will prevent unexpected transitions.
*/
lock_page(page);
shmem_swizzled = PageSwapCache(page) || page->mapping;
unlock_page(page);
put_page(page);
folio_lock(folio);
shmem_swizzled = folio_test_swapcache(folio) || folio->mapping;
folio_unlock(folio);
folio_put(folio);
if (shmem_swizzled)
goto again;
@ -331,14 +330,14 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
/*
* Private mappings are handled in a simple way.
*
* If the futex key is stored on an anonymous page, then the associated
* If the futex key is stored in anonymous memory, then the associated
* object is the mm which is implicitly pinned by the calling process.
*
* NOTE: When userspace waits on a MAP_SHARED mapping, even if
* it's a read-only handle, it's expected that futexes attach to
* the object not the particular process.
*/
if (PageAnon(page)) {
if (folio_test_anon(folio)) {
/*
* A RO anonymous page will never change and thus doesn't make
* sense for futex operations.
@ -357,10 +356,10 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
/*
* The associated futex object in this case is the inode and
* the page->mapping must be traversed. Ordinarily this should
* be stabilised under page lock but it's not strictly
* the folio->mapping must be traversed. Ordinarily this should
* be stabilised under folio lock but it's not strictly
* necessary in this case as we just want to pin the inode, not
* update the radix tree or anything like that.
* update i_pages or anything like that.
*
* The RCU read lock is taken as the inode is finally freed
* under RCU. If the mapping still matches expectations then the
@ -368,9 +367,9 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
*/
rcu_read_lock();
if (READ_ONCE(page->mapping) != mapping) {
if (READ_ONCE(folio->mapping) != mapping) {
rcu_read_unlock();
put_page(page);
folio_put(folio);
goto again;
}
@ -378,19 +377,19 @@ int get_futex_key(u32 __user *uaddr, bool fshared, union futex_key *key,
inode = READ_ONCE(mapping->host);
if (!inode) {
rcu_read_unlock();
put_page(page);
folio_put(folio);
goto again;
}
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
key->shared.i_seq = get_inode_sequence_number(inode);
key->shared.pgoff = page_to_pgoff(tail);
key->shared.pgoff = folio->index + folio_page_idx(folio, page);
rcu_read_unlock();
}
out:
put_page(page);
folio_put(folio);
return err;
}