netdev - Re: [PATCH net] page_pool: mask the page->signature before the checking

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Date:   Thu, 5 Aug 2021 10:14:39 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <davem@...emloft.net>, <kuba@...nel.org>, <hawk@...nel.org>,
        <ilias.apalodimas@...aro.org>, <mcroce@...rosoft.com>,
        <alexander.duyck@...il.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>,
        <chenhao288@...ilicon.com>
Subject: Re: [PATCH net] page_pool: mask the page->signature before the
 checking

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?

> .
>