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 kzpg0JHt hriJWMgUquaFOoseAf0t3PVAuXtBwMqQnSBN2Ig35ZIhAyhgyiAhHFx24ru90GrY37kZj1Z29y/r03AG9i2kjTnStXzIZQKz9H2enkNz8NUR5ZbxSx4YzKZjZmkSSBVE1sbD7DA81Xzklft66hQSi5ona7lRQdwK5qUJ2/H3SD5AjaKofjzqEjiuxbsFsHSH1fFbVYKhlwWpDikkYVVfbxUW1tMtGZkRTNG8tFzsoX5i4AAlXAlmRycycWYSM0UKlYzK83ioYIxp0ydGv754KQ7oUQSWiv1BJYNpQ3VsCI07iPO5mJfBwvNxdAvrPC2sJzPVEVzYeT89cE9Fd+1HT7TZhQjtQWTzzU99xCGjeABhTk9ir7WfrhEaaBcPcCmR9AL+XGMpK3PHWFaXswal3rHvPDsWSPgsal72dMM7ouJMujudH1IhWD95nldPk7u28frxiursMzqOxMpU= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000684, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 在 2024/8/25 14:42, Lorenzo Stoakes 写道: > [Some people who received this message don't often get email from lorenzo.stoakes@oracle.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ] > > On Sun, Aug 25, 2024 at 01:06:40PM GMT, zhiguojiang wrote: >> >> 在 2024/8/25 0:26, Lorenzo Stoakes 写道: >>> [Some people who received this message don't often get email from lorenzo.stoakes@oracle.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ] >>> >>> 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 >>> 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