On 2021/8/5 9:50, Matthew Wilcox wrote:
On Thu, Aug 05, 2021 at 09:06:57AM +0800, Yunsheng Lin wrote:
As mentioned in commit c07aea3ef4d4 ("mm: add a signature in struct page"): "The page->signature field is aliased to page->lru.next and page->compound_head."
And as the comment in page_is_pfmemalloc(): "lru.next has bit 1 set if the page is allocated from the pfmemalloc reserves. Callers may simply overwrite it if they do not need to preserve that information."
The page->signature is or’ed with PP_SIGNATURE when a page is allocated in page pool, see __page_pool_alloc_pages_slow(), and page->signature is checked directly with PP_SIGNATURE in page_pool_return_skb_page(), which might cause resoure leaking problem for a page from page pool if bit 1 of lru.next is set for a pfmemalloc page.
As bit 0 is page->compound_head, So mask both bit 0 and 1 before the checking in page_pool_return_skb_page().
No, you don't understand. We *want* the check to fail if we were low on memory so we return the emergency allocation.
If the check failed, but the page pool assume the page is not from page pool and will not do the resource cleaning(like dma unmapping), as the page pool still use the page with pfmemalloc set and dma map the page if pp_flags & PP_FLAG_DMA_MAP is true in __page_pool_alloc_pages_slow().
The returning the emergency allocation you mentioned seems to be handled in __page_pool_put_page(), see:
https://elixir.bootlin.com/linux/latest/source/net/core/page_pool.c#L411
We just use the page with pfmemalloc one time and do the resource cleaning before returning the page back to page allocator. Or did I miss something here?
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