netdev - Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE

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Date: Fri, 23 Jun 2023 10:06:59 +0100
From: David Howells <dhowells@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: dhowells@...hat.com, netdev@...r.kernel.org,
    Alexander Duyck <alexander.duyck@...il.com>,
    "David S. Miller" <davem@...emloft.net>,
    Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
    Willem de Bruijn <willemdebruijn.kernel@...il.com>,
    David Ahern <dsahern@...nel.org>,
    Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
    linux-mm@...ck.org, linux-kernel@...r.kernel.org,
    Menglong Dong <imagedong@...cent.com>
Subject: Re: [PATCH net-next v3 01/18] net: Copy slab data for sendmsg(MSG_SPLICE_PAGES)

Paolo Abeni <pabeni@...hat.com> wrote:

> IMHO this function uses a bit too much labels and would be more easy to
> read, e.g. moving the above chunk of code in conditional branch.

Maybe.  I was trying to put the fast path up at the top without the slow path
bits in it, but I can put the "insufficient_space" bit there.

> Even without such change, I think the above 'goto try_again;'
> introduces an unneeded conditional, as at this point we know 'fragsz <=
> fsize'.

Good point.

> > +		cache->pfmemalloc = folio_is_pfmemalloc(spare);
> > +		if (cache->folio)
> > +			goto reload;
> 
> I think there is some problem with the above.
> 
> If cache->folio is != NULL, and cache->folio was not pfmemalloc-ed
> while the spare one is, it looks like the wrong policy will be used.
> And should be even worse if folio was pfmemalloc-ed while spare is not.
> 
> I think moving 'cache->pfmemalloc' initialization...
> 
> > +	}
> > +
> 
> ... here should fix the above.

Yeah.  We might have raced with someone else or been moved to another cpu and
there might now be a folio we can allocate from.

> > +	/* Reset page count bias and offset to start of new frag */
> > +	cache->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
> > +	offset = folio_size(folio);
> > +	goto try_again;
> 
> What if fragsz > PAGE_SIZE, we are consistently unable to allocate an
> high order page, but order-0, pfmemalloc-ed page allocation is
> successful? It looks like this could become an unbounded loop?

It shouldn't.  It should go:

	try_again:
		if (fragsz > offset)
			goto insufficient_space;
	insufficient_space:
		/* See if we can refurbish the current folio. */
		...
		fsize = folio_size(folio);
		if (unlikely(fragsz > fsize))
			goto frag_too_big;
	frag_too_big:
		...
		return NULL;

Though for safety's sake, it would make sense to put in a size check in the
case we fail to allocate a larger-order folio.

> >  		do {
> >  			struct page *page = pages[i++];
> >  			size_t part = min_t(size_t, PAGE_SIZE - off, len);
> > -
> > -			ret = -EIO;
> > -			if (WARN_ON_ONCE(!sendpage_ok(page)))
> > +			bool put = false;
> > +
> > +			if (PageSlab(page)) {
> 
> I'm a bit concerned from the above. If I read correctly, tcp 0-copy

Well, splice()-to-tcp will; MSG_ZEROCOPY is unaffected.

> will go through that for every page, even if the expected use-case is
> always !PageSlub(page). compound_head() could be costly if the head
> page is not hot on cache and I'm not sure if that could be the case for
> tcp 0-copy. The bottom line is that I fear a possible regression here.

I can put the PageSlab() check inside the sendpage_ok() so the page flag is
only checked once.  But PageSlab() doesn't check the headpage, only the page
it is given.  sendpage_ok() is more the problem as it also calls
page_count().  I could drop the check.

David