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 11:00:26 +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:

> > 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.
> 
> I *think* you could move the insufficient_space in a separate helped,
> that should achieve your goal with fewer labels and hopefully no
> additional complexity.

It would require moving things in and out of variables more, but that's
probably fine in the slow path.  The code I derived this from seems to do its
best only to touch memory when it absolutely has to.  But doing what you
suggest would certainly make this more readable, I think.

> > > 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. */
> > 		...
> 
> I think the critical path is with pfmemalloc-ed pages:
> 
> 		if (unlikely(cache->pfmemalloc)) {
> 			__folio_put(folio);
> 			goto get_new_folio;
> 		}

I see what you're getting at.  I was thinking that you meant that the critical
bit was that we got into a loop because we never managed to allocate a folio
big enough.

Inserting a check in the event that we fail to allocate an order-3 folio would
take care of that, I think.  After that point, we have a spare folio of
sufficient capacity, even if the folio currently in residence is marked
pfmemalloc.

> > > 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.  
> 
> Perhaps I'm lost again, but AFAICS:
> 
> __PAGEFLAG(Slab, slab, PF_NO_TAIL)
> ...
>                 PF_POISONED_CHECK(compound_head(page)); })
> 
> It looks at compound_head in the end ?!?

Fair point.  Those macros are somewhat hard to read.  Hopefully, all the
compound_head() calls will go away soon-ish.