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zsmalloc: fix races between asynchronous zspage free and page migration

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The asynchronous zspage free worker tries to lock a zspage's entire page
list without defending against page migration.  Since pages which haven't
yet been locked can concurrently migrate off the zspage page list while
lock_zspage() churns away, lock_zspage() can suffer from a few different
lethal races.

It can lock a page which no longer belongs to the zspage and unsafely
dereference page_private(), it can unsafely dereference a torn pointer to
the next page (since there's a data race), and it can observe a spurious
NULL pointer to the next page and thus not lock all of the zspage's pages
(since a single page migration will reconstruct the entire page list, and
create_page_chain() unconditionally zeroes out each list pointer in the
process).

Fix the races by using migrate_read_lock() in lock_zspage() to synchronize
with page migration.

Link: https://lkml.kernel.org/r/20220509024703.243847-1-sultan@kerneltoast.com
Fixes: 77ff465799 ("zsmalloc: zs_page_migrate: skip unnecessary loops but not return -EBUSY if zspage is not inuse")
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Sultan Alsawaf 2022-05-13 15:11:26 -07:00 committed by Andrew Morton
parent 60a60e32cf
commit 2505a98111
1 changed files with 33 additions and 4 deletions
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37
mm/zsmalloc.c
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@ -1718,11 +1718,40 @@ static enum fullness_group putback_zspage(struct size_class *class,
*/
static void lock_zspage(struct zspage *zspage)
{
struct page *page = get_first_page(zspage);
struct page *curr_page, *page;
do {
lock_page(page);
} while ((page = get_next_page(page)) != NULL);
/*
* Pages we haven't locked yet can be migrated off the list while we're
* trying to lock them, so we need to be careful and only attempt to
* lock each page under migrate_read_lock(). Otherwise, the page we lock
* may no longer belong to the zspage. This means that we may wait for
* the wrong page to unlock, so we must take a reference to the page
* prior to waiting for it to unlock outside migrate_read_lock().
*/
while (1) {
migrate_read_lock(zspage);
page = get_first_page(zspage);
if (trylock_page(page))
break;
get_page(page);
migrate_read_unlock(zspage);
wait_on_page_locked(page);
put_page(page);
}
curr_page = page;
while ((page = get_next_page(curr_page))) {
if (trylock_page(page)) {
curr_page = page;
} else {
get_page(page);
migrate_read_unlock(zspage);
wait_on_page_locked(page);
put_page(page);
migrate_read_lock(zspage);
}
}
migrate_read_unlock(zspage);
}
static int zs_init_fs_context(struct fs_context *fc)