From: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
To: "Liam.Howlett@Oracle.com" <Liam.Howlett@Oracle.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [PATCH 3/3] mm/mmap: Regression fix for unmapped_area{_topdown}
Date: Fri, 14 Apr 2023 18:21:53 +0000 [thread overview]
Message-ID: <5f15e46556ac4366682777f797157e48620c3b05.camel@intel.com> (raw)
In-Reply-To: <20230414180759.zves3glmrptl6zog@revolver>
On Fri, 2023-04-14 at 14:07 -0400, Liam R. Howlett wrote:
> * Edgecombe, Rick P <rick.p.edgecombe@intel.com> [230414 13:53]:
> > On Fri, 2023-04-14 at 13:29 -0400, Liam R. Howlett wrote:
> > > * Liam R. Howlett <Liam.Howlett@Oracle.com> [230414 13:26]:
> > > > * Edgecombe, Rick P <rick.p.edgecombe@intel.com> [230414
> > > > 12:27]:
> > > > > On Fri, 2023-04-14 at 10:57 -0400, Liam R. Howlett wrote:<br>
> > > > > > + tmp = mas_next(&mas, ULONG_MAX);
> > > > > > + if (tmp && (tmp->vm_flags & VM_GROWSDOWN)) {
> > > > >
> > > > > Why also check VM_GROWSDOWN here (and VM_GROWSUP below)?
> > > > > vm_start/end_gap() already have checks inside.
> > > >
> > > > An artifact of a plan that was later abandoned.
> > > >
> > > > >
> > > > > > + if (vm_start_gap(tmp) < gap + length - 1) {
> > > > > > + low_limit = tmp->vm_end;
> > > > > > + mas_reset(&mas);
> > > > > > + goto retry;
> > > > > > + }
> > > > > > + } else {
> > > > > > + tmp = mas_prev(&mas, 0);
> > > > > > + if (tmp && (tmp->vm_flags & VM_GROWSUP) &&
> > > > > > + vm_end_gap(tmp) > gap) {
> > > > > > + low_limit = vm_end_gap(tmp);
> > > > > > + mas_reset(&mas);
> > > > > > + goto retry;
> > > > > > + }
> > > > > > + }
> > > > > > +
> > > > >
> > > > > Could it be like this?
> > > >
> > > > Yes, I'll make this change. Thanks for the suggestion.
> > >
> > >
> > > Wait, I like how it is.
> > >
> > > In my version, if there is a stack that is VM_GROWSDOWN there,
> > > but
> > > does
> > > not intercept the gap, then I won't check the prev.. in yours, we
> > > will
> > > never avoid checking prev.
> >
> > Hmm, I see. I guess I'm thinking ahead a bit to adding the shadow
> > stack
> > guard gap, but I can always add to these vm_flags checks.
> >
> > But are you sure this optimization is even possible? The old
> > vma_compute_gap() had this comment:
> > /*
> > * Note: in the rare case of a VM_GROWSDOWN above a VM_GROWSUP, we
> > * allow two stack_guard_gaps between them here, and when choosing
> > * an unmapped area; whereas when expanding we only require one.
> > * That's a little inconsistent, but keeps the code here simpler.
> > */
>
> I didn't think this was possible. ia64 (orphaned in 96ec72a3425d)
> did
> this.
Ah, ok.
>
> >
> > Assuming this is a real scenario, if you have VM_GROWSDOWN above
> > and
> > VM_GROWSUP below, don't you need to check the gaps for above and
> > below?
> > Again thinking about adding shadow stack guard pages, something
> > like
> > that could be a more common scenario. Not that you need to fix my
> > out
> > of tree issues, but I would probably need to adjust it to check
> > both
> > directions.
> >
> > I guess there is no way to embed this inside maple tree search so
> > we
> > don't need to retry? (sorry if this is a dumb question, it's an
> > opaque
> > box to me).
>
> Absolutely, and I'm working on this as well, but right now I'm trying
> to fix my regression for this and past releases. Handling this in
> the
> maple tree is more involved and so there's more risk.
Ok, thanks. It looks good to me in that respect.
next prev parent reply other threads:[~2023-04-14 18:22 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-14 14:57 [PATCH 1/3] maple_tree: Make maple state reusable after mas_empty_area_rev() Liam R. Howlett
2023-04-14 14:57 ` [PATCH 2/3] maple_tree: Fix mas_empty_area() search Liam R. Howlett
2023-04-14 14:57 ` [PATCH 3/3] mm/mmap: Regression fix for unmapped_area{_topdown} Liam R. Howlett
2023-04-14 16:27 ` Edgecombe, Rick P
2023-04-14 17:26 ` Liam R. Howlett
2023-04-14 17:29 ` Liam R. Howlett
2023-04-14 17:53 ` Edgecombe, Rick P
2023-04-14 18:07 ` Liam R. Howlett
2023-04-14 18:21 ` Edgecombe, Rick P [this message]
2023-04-14 18:59 ` [PATCH v2] " Liam R. Howlett
2023-04-14 19:09 ` Andrew Morton
2023-04-29 14:32 ` Tad
2023-04-30 22:41 ` Michael Keyes
2023-05-02 14:08 ` Liam R. Howlett
2023-05-02 14:09 ` Liam R. Howlett
2023-05-15 8:57 ` Juhyung Park
2023-05-15 14:36 ` Liam R. Howlett
2023-04-19 9:02 ` [PATCH 1/3] maple_tree: Make maple state reusable after mas_empty_area_rev() Peng Zhang
2023-04-19 22:54 ` Liam R. Howlett
2023-04-20 4:21 ` Peng Zhang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5f15e46556ac4366682777f797157e48620c3b05.camel@intel.com \
--to=rick.p.edgecombe@intel.com \
--cc=Liam.Howlett@Oracle.com \
--cc=akpm@linux-foundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).