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mm/memfd: refactor memfd_tag_pins() and memfd_wait_for_pins()

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Patch series "mm: remove total_mapcount()", v2.

Let's remove the remaining user from mm/memfd.c so we can get rid of
total_mapcount().


This patch (of 2):

Both functions are the remaining users of total_mapcount().  Let's get rid
of the calls by converting the code to folios.

As it turns out, the code is unnecessarily complicated, especially:

1) We can query the number of pagecache references for a folio simply via
   folio_nr_pages(). This will handle other folio sizes in the future
   correctly.

2) The xas_set(xas, page->index + cache_count) call to increment the
   iterator for large folios is not required. Remove it.

Further, simplify the XA_CHECK_SCHED check, counting each entry exactly
once.

Memfd pages can be swapped out when using shmem; leave xa_is_value()
checks in place.

Link: https://lkml.kernel.org/r/20240226141324.278526-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240226141324.278526-2-david@redhat.com
Co-developed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2024-02-26 15:13:23 +01:00 committed by Andrew Morton
parent fc4d182316
commit b4d02baa9f
1 changed files with 18 additions and 29 deletions
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47
mm/memfd.c
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@ -29,29 +29,25 @@
#define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE
#define LAST_SCAN 4 /* about 150ms max */
static bool memfd_folio_has_extra_refs(struct folio *folio)
{
return folio_ref_count(folio) - folio_mapcount(folio) !=
folio_nr_pages(folio);
}
static void memfd_tag_pins(struct xa_state *xas)
{
struct page *page;
struct folio *folio;
int latency = 0;
int cache_count;
lru_add_drain();
xas_lock_irq(xas);
xas_for_each(xas, page, ULONG_MAX) {
cache_count = 1;
if (!xa_is_value(page) &&
PageTransHuge(page) && !PageHuge(page))
cache_count = HPAGE_PMD_NR;
if (!xa_is_value(page) &&
page_count(page) - total_mapcount(page) != cache_count)
xas_for_each(xas, folio, ULONG_MAX) {
if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
xas_set_mark(xas, MEMFD_TAG_PINNED);
if (cache_count != 1)
xas_set(xas, page->index + cache_count);
latency += cache_count;
if (latency < XA_CHECK_SCHED)
if (++latency < XA_CHECK_SCHED)
continue;
latency = 0;
@ -66,16 +62,16 @@ static void memfd_tag_pins(struct xa_state *xas)
/*
* Setting SEAL_WRITE requires us to verify there's no pending writer. However,
* via get_user_pages(), drivers might have some pending I/O without any active
* user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
* user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
* and see whether it has an elevated ref-count. If so, we tag them and wait for
* them to be dropped.
* The caller must guarantee that no new user will acquire writable references
* to those pages to avoid races.
* to those folios to avoid races.
*/
static int memfd_wait_for_pins(struct address_space *mapping)
{
XA_STATE(xas, &mapping->i_pages, 0);
struct page *page;
struct folio *folio;
int error, scan;
memfd_tag_pins(&xas);
@ -83,7 +79,6 @@ static int memfd_wait_for_pins(struct address_space *mapping)
error = 0;
for (scan = 0; scan <= LAST_SCAN; scan++) {
int latency = 0;
int cache_count;
if (!xas_marked(&xas, MEMFD_TAG_PINNED))
break;
@ -95,20 +90,15 @@ static int memfd_wait_for_pins(struct address_space *mapping)
xas_set(&xas, 0);
xas_lock_irq(&xas);
xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
bool clear = true;
cache_count = 1;
if (!xa_is_value(page) &&
PageTransHuge(page) && !PageHuge(page))
cache_count = HPAGE_PMD_NR;
if (!xa_is_value(page) && cache_count !=
page_count(page) - total_mapcount(page)) {
if (!xa_is_value(folio) &&
memfd_folio_has_extra_refs(folio)) {
/*
* On the last scan, we clean up all those tags
* we inserted; but make a note that we still
* found pages pinned.
* found folios pinned.
*/
if (scan == LAST_SCAN)
error = -EBUSY;
@ -118,8 +108,7 @@ static int memfd_wait_for_pins(struct address_space *mapping)
if (clear)
xas_clear_mark(&xas, MEMFD_TAG_PINNED);
latency += cache_count;
if (latency < XA_CHECK_SCHED)
if (++latency < XA_CHECK_SCHED)
continue;
latency = 0;