Re: [PATCH v2] vma remove the unneeded avc bound with non-CoWed folio

[zhiguojiang]

在 2024/8/25 14:42, Lorenzo Stoakes 写道:
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> On Sun, Aug 25, 2024 at 01:06:40PM GMT, zhiguojiang wrote:
> > 在 2024/8/25 0:26, Lorenzo Stoakes 写道:
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> > >
> > > On Fri, Aug 23, 2024 at 11:02:06PM GMT, Zhiguo Jiang wrote:
> > > > After CoWed by do_wp_page, the vma established a new mapping relationship
> > > > with the CoWed folio instead of the non-CoWed folio. However, regarding
> > > > the situation where vma->anon_vma and the non-CoWed folio's anon_vma are
> > > > not same, the avc binding relationship between them will no longer be
> > > > needed, so it is issue for the avc binding relationship still existing
> > > > between them.
> > > >
> > > > This patch will remove the avc binding relationship between vma and the
> > > > non-CoWed folio's anon_vma, which each has their own independent
> > > > anon_vma. It can also alleviates rmap overhead simultaneously.
> > > >
> > > > Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
> > > NACK (until fixed). This is broken (see below).
> > Hi Lorenzo Stoakes,
> >
> > Thank you for your comments.
> > > I'm not seeing any numbers to back anything up here as to why we want to
> > > make changes to this incredibly sensitive code?
> > I added a debug trace log (as follows) in wp_page_copy() and observed
> > that a large number of these orphan avc-objects still exist. I believe
> > this will have a certain redundant overhead impact on anonymous folios'
> > rmap avcs, so I want to remove it, which is also the most essential
> > value of this patch.
> Sorry nack to that idea unless you can provide actual _data_ to demonstrate
> an overhead.
>
> And even if you did, given the original patch was so completely broken, and
> in such a sensitive area, I'm going to need to be VERY confident you didn't
> break anything, so we're going to need tests.
>
> > -- the vital part of debug trace patch:
> Thanks for providing! Will snip for sake of making it easier to reply.
>
> [snip]
>
> > > Also anon_vma logic is very complicated and confusing, this commit message
> > > feels about 3 paragraphs too light.
> > >
> > > Under what circumstances will vma->anon_vma be different from
> > > folio_anon_vma(non_cowed_folio)? etc.
> > In anon_vma_fork() --> anon_vma_clone(), child vma is bound with parent
> > vma's anon_vma firstly.
> >      /*
> >       * First, attach the new VMA to the parent VMA's anon_vmas,
> >       * so rmap can find non-COWed pages in child processes.
> >       */
> >      error = anon_vma_clone(vma, pvma);
> >
> > When child vma->anon_vma is NULL in anon_vma_fork(),
> >      /* An existing anon_vma has been reused, all done then. */
> >      if (vma->anon_vma)
> >          return 0;
> >
> >      /* Then add our own anon_vma. */
> >      anon_vma = anon_vma_alloc();
> >
> > new anon_vma will be alloced and filled in this child vma->anon_vma.
> > Then during CoWed in do_wp_page() --> wp_page_copy(), this child vma's
> > new anon_vma will be different from folio_anon_vma(non_cowed_folio).
> Thanks for the explanation, but I was suggesting you have to put this in
> the commit message rather than in repy to me :)
Ok, I will update it in next version patch.
> > > Confusing topics strongly require explanations that help (somewhat)
> > > compensate. This is one of them.
> [snip]
>
> > > > index 93c0c25433d0..4c89cb1cb73e
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -3428,6 +3428,14 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> > > >                          * old page will be flushed before it can be reused.
> > > >                          */
> > > >                         folio_remove_rmap_pte(old_folio, vmf->page, vma);
> > > > +
> > > > +                     /*
> > > > +                      * If the new_folio's anon_vma is different from the
> > > > +                      * old_folio's anon_vma, the avc binding relationship
> > > > +                      * between vma and the old_folio's anon_vma is removed,
> > > > +                      * avoiding rmap redundant overhead.
> > > What overhead? Worth spelling out for instance if it's unnecessary to
> > > traverse avc's.
> > I think this will have a certain redundant overhead impact on anonymous
> > folios rmap traverse avcs process.
> This is again nowhere near detailed enough, and again I'm asking you to
> write this _IN THE COMMENT_ not in review.
Ok, I will try to test and update it in next version patch.
> I already understand what you're trying to do (I think the fact I provided
> a _working_ version of your patch as an attachment in this thread should
> give a clue ;), this is for the benefit of people coming to read this code.
>
> [snip]
>
> > > Again I question the value of this change. Are we REALLY seeing a big
> > > problem due to unneeded avc's hanging around? This is very sensitive,
> > > fiddly, confusing code, do we REALLY want to be playing with it?
> > Thank you for helping to identify mang issues with this patch. However,
> > I think this will have a certain benefits for anonymous folio rmap
> > traverse avc overhead.
> > > It'd be good to get some tests though unless you move this to vma.c with
> > > its userland testing (probably a good idea actually as Andrew suggested)
> > > this might be tricky.
> > This patch belongs to anon_vma rmap's content, and it seems more
> > appropriate in mm/rmap.c?
> > > NACK until the issues are fixed and the approach at least seems more
> > > correct.
> > Thanks
> > Zhiguo
> Please see the attachment in thread for an example of a working version of
> this, this is sadly fundamentally broken.
Yes, I have seen the attachment you provided and thank you very much for
your help. I will update it based on your modifications in next version.
Currently I am thinking about how to implement the anon_vma->root code.
> But you're going to really need to sell this a lot better, provide some
> numbers, and provide extensive testing and a much, much better test for
> this to stand any chance.
Ok, I will try to do some tests.
> I appreciate what you're trying to do here, and it's not totally crazy, but
> we have to be so, so careful around this code.
>
> anon_vma code is horrendously subtle and confusing (I actually had to
> reference my unpublished book to remind myself how this stuff works :)), so
> we have to tread very carefully.
You are right, anon_vma code is indeed complex and sensitive.
> I definitely think we need ASCII diagrams if we were to go ahead with a new
> version of this. But then again I'm a bit of a fan of ASCII diagrams...
>
> Please cc- me on future revisions of this series, thanks :)
Ok.

Thanks
Zhiguo