Commit 4c450583 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller

net: fix races in page->_count manipulation

This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.

The only case it is valid is when page->_count is 0

Fixes: 540eb7bf ("net: Update alloc frag to reduce get/put page usage and recycle pages")
Signed-off-by: default avatarEric Dumaze <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 98226208
......@@ -360,18 +360,29 @@ static void *__netdev_alloc_frag(unsigned int fragsz, gfp_t gfp_mask)
goto end;
}
nc->frag.size = PAGE_SIZE << order;
recycle:
atomic_set(&nc->frag.page->_count, NETDEV_PAGECNT_MAX_BIAS);
/* Even if we own the page, we do not use atomic_set().
* This would break get_page_unless_zero() users.
*/
atomic_add(NETDEV_PAGECNT_MAX_BIAS - 1,
&nc->frag.page->_count);
nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
nc->frag.offset = 0;
}
if (nc->frag.offset + fragsz > nc->frag.size) {
/* avoid unnecessary locked operations if possible */
if ((atomic_read(&nc->frag.page->_count) == nc->pagecnt_bias) ||
atomic_sub_and_test(nc->pagecnt_bias, &nc->frag.page->_count))
goto recycle;
if (atomic_read(&nc->frag.page->_count) != nc->pagecnt_bias) {
if (!atomic_sub_and_test(nc->pagecnt_bias,
&nc->frag.page->_count))
goto refill;
/* OK, page count is 0, we can safely set it */
atomic_set(&nc->frag.page->_count,
NETDEV_PAGECNT_MAX_BIAS);
} else {
atomic_add(NETDEV_PAGECNT_MAX_BIAS - nc->pagecnt_bias,
&nc->frag.page->_count);
}
nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;
nc->frag.offset = 0;
}
data = page_address(nc->frag.page) + nc->frag.offset;
......
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