Re: [PATCH 1/1] mm: avoid re-using pfmemalloc page in page_frag_alloc()

[Rama Nichanamatlu]

> Thanks for providing the numbers.  Do you think that dropping (up to)
> 7 packets is acceptable?

net.ipv4.tcp_syn_retries = 6

tcp clients wouldn't even get that far leading to connect establish issues.

-rama
On Tue, Nov 03, 2020 at 09:15:41PM +0000, Matthew Wilcox wrote:
> On Tue, Nov 03, 2020 at 12:57:33PM -0800, Dongli Zhang wrote:
> > On 11/3/20 12:35 PM, Matthew Wilcox wrote:
> > > On Tue, Nov 03, 2020 at 11:32:39AM -0800, Dongli Zhang wrote:
> > >> However, once kernel is not under memory pressure any longer (suppose large
> > >> amount of memory pages are just reclaimed), the page_frag_alloc() may still
> > >> re-use the prior pfmemalloc page_frag_cache->va to allocate skb->data. As a
> > >> result, the skb->pfmemalloc is always true unless page_frag_cache->va is
> > >> re-allocated, even the kernel is not under memory pressure any longer.
> > >> +	/*
> > >> +	 * Try to avoid re-using pfmemalloc page because kernel may already
> > >> +	 * run out of the memory pressure situation at any time.
> > >> +	 */
> > >> +	if (unlikely(nc->va && nc->pfmemalloc)) {
> > >> +		page = virt_to_page(nc->va);
> > >> +		__page_frag_cache_drain(page, nc->pagecnt_bias);
> > >> +		nc->va = NULL;
> > >> +	}
> > >
> > > I think this is the wrong way to solve this problem.  Instead, we should
> > > use up this page, but refuse to recycle it.  How about something like this (not even compile tested):
> >
> > Thank you very much for the feedback. Yes, the option is to use the same page
> > until it is used up (offset < 0). Instead of recycling it, the kernel free it
> > and allocate new one.
> >
> > This depends on whether we will tolerate the packet drop until this page is used up.
> >
> > For virtio-net, the payload (skb->data) is of size 128-byte. The padding and
> > alignment will finally make it as 512-byte.
> >
> > Therefore, for virtio-net, we will have at most 4096/512-1=7 packets dropped
> > before the page is used up.
>
> My thinking is that if the kernel is under memory pressure then freeing
> the page and allocating a new one is likely to put even more strain
> on the memory allocator, so we want to do this "soon", rather than at
> each allocation.
>
> Thanks for providing the numbers.  Do you think that dropping (up to)
> 7 packets is acceptable?
>
> We could also do something like ...
>
>        if (unlikely(nc->pfmemalloc)) {
>                page = alloc_page(GFP_NOWAIT | __GFP_NOWARN);
>                if (page)
>                        nc->pfmemalloc = 0;
>                put_page(page);
>        }
>
> to test if the memory allocator has free pages at the moment.  Not sure
> whether that's a good idea or not -- hopefully you have a test environment
> set up where you can reproduce this condition on demand and determine
> which of these three approaches is best!