[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZNZarsR7cVn/QH+H@x1n>
Date: Fri, 11 Aug 2023 11:58:38 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
Ryan Roberts <ryan.roberts@....com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-doc@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Jonathan Corbet <corbet@....net>,
Mike Kravetz <mike.kravetz@...cle.com>,
Hugh Dickins <hughd@...gle.com>,
Yin Fengwei <fengwei.yin@...el.com>,
Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios
On Fri, Aug 11, 2023 at 05:32:37PM +0200, David Hildenbrand wrote:
> On 11.08.23 17:18, Peter Xu wrote:
> > On Fri, Aug 11, 2023 at 12:27:13AM +0200, David Hildenbrand wrote:
> > > On 10.08.23 23:48, Matthew Wilcox wrote:
> > > > On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
> > > > > AFAICS if that patch was all correct (while I'm not yet sure..), you can
> > > > > actually fit your new total mapcount field into page 1 so even avoid the
> > > > > extra cacheline access. You can have a look: the trick is refcount for
> > > > > tail page 1 is still seems to be free on 32 bits (if that was your worry
> > > > > before). Then it'll be very nice if to keep Hugh's counter all in tail 1.
> > > >
> > > > No, refcount must be 0 on all tail pages. We rely on this in many places
> > > > in the MM.
> > >
> > > Very right.
> >
> > Obviously I could have missed this in the past.. can I ask for an example
> > explaining why refcount will be referenced before knowing it's a head?
>
> I think the issue is, when coming from a PFN walker (or GUP-fast), you might
> see "oh, this is a folio, let's lookup the head page". And you do that.
>
> Then, you try taking a reference on that head page. (see try_get_folio()).
>
> But as you didn't hold a reference on the folio yet, it can happily get
> freed + repurposed in the meantime, so maybe it's not a head page anymore.
>
> So if the field would get reused for something else, grabbing a reference
> would corrupt whatever is now stored in there.
Not an issue before large folios, am I right? Because having a head page
reused as tail cannot happen iiuc with current thps if only pmd-sized,
because the head page is guaranteed to be pmd aligned physically.
I don't really know, where a hugetlb 2M head can be reused by a 1G huge
later right during the window of fast-gup walking. But obviously that's not
common either if that could ever happen.
Maybe Matthew was referring to something else (per "in many places")?
Thanks,
--
Peter Xu
Powered by blists - more mailing lists