From: Daniel Forrest <dan.forrest@ssec.wisc.edu> To: Andrew Morton <akpm@linux-foundation.org> Cc: Rik van Riel <riel@redhat.com>, Michel Lespinasse <walken@google.com>, Hugh Dickins <hughd@google.com>, Andrea Arcangeli <aarcange@redhat.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Tim Hartrick <tim@edgecast.com>, Michal Hocko <mhocko@suse.cz> Subject: Re: [PATCH] Repeated fork() causes SLAB to grow without bound Date: Mon, 17 Nov 2014 19:41:35 -0600 [thread overview] Message-ID: <20141118014135.GA17252@cosmos.ssec.wisc.edu> (raw) In-Reply-To: <20141117160212.b86d031e1870601240b0131d@linux-foundation.org> On Mon, Nov 17, 2014 at 04:02:12PM -0800, Andrew Morton wrote: > On Fri, 14 Nov 2014 10:30:53 -0600 Daniel Forrest <dan.forrest@ssec.wisc.edu> wrote: > > > There have been a couple of inquiries about the status of this patch > > over the last few months, so I am going to try pushing it out. > > > > Andrea Arcangeli has commented: > > > > > Agreed. The only thing I don't like about this patch is the hardcoding > > > of number 5: could we make it a variable to tweak with sysfs/sysctl so > > > if some weird workload arises we have a tuning tweak? It'd cost one > > > cacheline during fork, so it doesn't look excessive overhead. > > > > Adding this is beyond my experience level, so if it is required then > > someone else will have to make it so. > > > > Rik van Riel has commented: > > > > > I believe we should just merge that patch. > > > > > > I have not seen any better ideas come by. > > > > > > The comment should probably be fixed to reflect the > > > chain length of 5 though :) > > > > So here is Michel's patch again with "(length > 1)" modified to > > "(length > 5)" and fixed comments. > > > > I have been running with this patch (with the threshold set to 5) for > > over two years now and it does indeed solve the problem. > > > > --- > > > > anon_vma_clone() is modified to return the length of the existing > > same_vma anon vma chain, and we create a new anon_vma in the child > > if it is more than five forks after the anon_vma was created, as we > > don't want the same_vma chain to grow arbitrarily large. > > hoo boy, what's going on here. > > - Under what circumstances are we seeing this slab windup? The original bug report is here: https://lkml.org/lkml/2012/8/15/765 > - What are the consequences? Can it OOM the machine? Yes, eventually you run out of SLAB space. > - Why is this occurring? There aren't an infinite number of vmas, so > there shouldn't be an infinite number of anon_vmas or > anon_vma_chains. Because of the serial forking there does indeed end up being an infinite number of vmas. The initial vma can never be deleted (even though the initial parent process has long since terminated) because the initial vma is referenced by the children. > - IOW, what has to be done to fix this properly? As far as I know, this is the best solution. I tried a refcounting solution based on comments by Rik van Riel: https://lkml.org/lkml/2012/8/17/536 But it didn't fully work, probably because I didn't quite get the locking done properly. In any case, at this point questions came up about the overhead of the page refcounting and Michel Lespinasse suggested the initial version of this patch: https://lkml.org/lkml/2012/8/21/730 > - What are the runtime consequences of limiting the length of the chain? I can't say, but it only affects users who fork more than five levels deep without doing an exec. On the other hand, there are at least three users (Tim Hartrick, Michal Hocko, and myself) who have real world applications where the consequence of no patch is a crashed system. I would suggest reading the thread starting with my initial bug report for what others have had to say about this. > > ... > > > > @@ -331,10 +334,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) > > * First, attach the new VMA to the parent VMA's anon_vmas, > > * so rmap can find non-COWed pages in child processes. > > */ > > - if (anon_vma_clone(vma, pvma)) > > + length = anon_vma_clone(vma, pvma); > > + if (length < 0) > > return -ENOMEM; > > This should propagate the anon_vma_clone() return val instead of > assuming ENOMEM. But that won't fix anything... Agreed, but the only failure return value of anon_vma_clone is -ENOMEM. Scanning the code in __split_vma (mm/mmap.c) it looks like the error return is lost (between Linux 3.11 and 3.12 the err variable is now used before the call to anon_vma_clone and the default initial value of -ENOMEM is overwritten). This is an actual bug in the current code. I can update the patch to fix these issues. > > + else if (length > 5) > > + return 0; > > > > - /* Then add our own anon_vma. */ > > + /* > > + * Then add our own anon_vma. We do this only for five forks after > > + * the anon_vma was created, as we don't want the same_vma chain to > > + * grow arbitrarily large. > > + */ > > anon_vma = anon_vma_alloc(); -- Daniel K. Forrest Space Science and dan.forrest@ssec.wisc.edu Engineering Center (608) 890 - 0558 University of Wisconsin, Madison
WARNING: multiple messages have this Message-ID (diff)
From: Daniel Forrest <dan.forrest@ssec.wisc.edu> To: Andrew Morton <akpm@linux-foundation.org> Cc: Rik van Riel <riel@redhat.com>, Michel Lespinasse <walken@google.com>, Hugh Dickins <hughd@google.com>, Andrea Arcangeli <aarcange@redhat.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Tim Hartrick <tim@edgecast.com>, Michal Hocko <mhocko@suse.cz> Subject: Re: [PATCH] Repeated fork() causes SLAB to grow without bound Date: Mon, 17 Nov 2014 19:41:35 -0600 [thread overview] Message-ID: <20141118014135.GA17252@cosmos.ssec.wisc.edu> (raw) In-Reply-To: <20141117160212.b86d031e1870601240b0131d@linux-foundation.org> On Mon, Nov 17, 2014 at 04:02:12PM -0800, Andrew Morton wrote: > On Fri, 14 Nov 2014 10:30:53 -0600 Daniel Forrest <dan.forrest@ssec.wisc.edu> wrote: > > > There have been a couple of inquiries about the status of this patch > > over the last few months, so I am going to try pushing it out. > > > > Andrea Arcangeli has commented: > > > > > Agreed. The only thing I don't like about this patch is the hardcoding > > > of number 5: could we make it a variable to tweak with sysfs/sysctl so > > > if some weird workload arises we have a tuning tweak? It'd cost one > > > cacheline during fork, so it doesn't look excessive overhead. > > > > Adding this is beyond my experience level, so if it is required then > > someone else will have to make it so. > > > > Rik van Riel has commented: > > > > > I believe we should just merge that patch. > > > > > > I have not seen any better ideas come by. > > > > > > The comment should probably be fixed to reflect the > > > chain length of 5 though :) > > > > So here is Michel's patch again with "(length > 1)" modified to > > "(length > 5)" and fixed comments. > > > > I have been running with this patch (with the threshold set to 5) for > > over two years now and it does indeed solve the problem. > > > > --- > > > > anon_vma_clone() is modified to return the length of the existing > > same_vma anon vma chain, and we create a new anon_vma in the child > > if it is more than five forks after the anon_vma was created, as we > > don't want the same_vma chain to grow arbitrarily large. > > hoo boy, what's going on here. > > - Under what circumstances are we seeing this slab windup? The original bug report is here: https://lkml.org/lkml/2012/8/15/765 > - What are the consequences? Can it OOM the machine? Yes, eventually you run out of SLAB space. > - Why is this occurring? There aren't an infinite number of vmas, so > there shouldn't be an infinite number of anon_vmas or > anon_vma_chains. Because of the serial forking there does indeed end up being an infinite number of vmas. The initial vma can never be deleted (even though the initial parent process has long since terminated) because the initial vma is referenced by the children. > - IOW, what has to be done to fix this properly? As far as I know, this is the best solution. I tried a refcounting solution based on comments by Rik van Riel: https://lkml.org/lkml/2012/8/17/536 But it didn't fully work, probably because I didn't quite get the locking done properly. In any case, at this point questions came up about the overhead of the page refcounting and Michel Lespinasse suggested the initial version of this patch: https://lkml.org/lkml/2012/8/21/730 > - What are the runtime consequences of limiting the length of the chain? I can't say, but it only affects users who fork more than five levels deep without doing an exec. On the other hand, there are at least three users (Tim Hartrick, Michal Hocko, and myself) who have real world applications where the consequence of no patch is a crashed system. I would suggest reading the thread starting with my initial bug report for what others have had to say about this. > > ... > > > > @@ -331,10 +334,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) > > * First, attach the new VMA to the parent VMA's anon_vmas, > > * so rmap can find non-COWed pages in child processes. > > */ > > - if (anon_vma_clone(vma, pvma)) > > + length = anon_vma_clone(vma, pvma); > > + if (length < 0) > > return -ENOMEM; > > This should propagate the anon_vma_clone() return val instead of > assuming ENOMEM. But that won't fix anything... Agreed, but the only failure return value of anon_vma_clone is -ENOMEM. Scanning the code in __split_vma (mm/mmap.c) it looks like the error return is lost (between Linux 3.11 and 3.12 the err variable is now used before the call to anon_vma_clone and the default initial value of -ENOMEM is overwritten). This is an actual bug in the current code. I can update the patch to fix these issues. > > + else if (length > 5) > > + return 0; > > > > - /* Then add our own anon_vma. */ > > + /* > > + * Then add our own anon_vma. We do this only for five forks after > > + * the anon_vma was created, as we don't want the same_vma chain to > > + * grow arbitrarily large. > > + */ > > anon_vma = anon_vma_alloc(); -- Daniel K. Forrest Space Science and dan.forrest@ssec.wisc.edu Engineering Center (608) 890 - 0558 University of Wisconsin, Madison -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2014-11-18 1:41 UTC|newest] Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-08-16 2:46 Repeated fork() causes SLAB to grow without bound Daniel Forrest 2012-08-16 18:58 ` Rik van Riel 2012-08-16 18:58 ` Rik van Riel 2012-08-18 0:03 ` Daniel Forrest 2012-08-18 0:03 ` Daniel Forrest 2012-08-18 3:46 ` Rik van Riel 2012-08-18 3:46 ` Rik van Riel 2012-08-18 4:07 ` Daniel Forrest 2012-08-18 4:07 ` Daniel Forrest 2012-08-18 4:10 ` Rik van Riel 2012-08-18 4:10 ` Rik van Riel 2012-08-20 8:00 ` Hugh Dickins 2012-08-20 8:00 ` Hugh Dickins 2012-08-20 9:39 ` Michel Lespinasse 2012-08-20 9:39 ` Michel Lespinasse 2012-08-20 11:11 ` Andi Kleen 2012-08-20 11:11 ` Andi Kleen 2012-08-20 11:17 ` Rik van Riel 2012-08-20 11:17 ` Rik van Riel 2012-08-20 11:53 ` Michel Lespinasse 2012-08-20 11:53 ` Michel Lespinasse 2012-08-20 19:11 ` Michel Lespinasse 2012-08-20 19:11 ` Michel Lespinasse 2012-08-22 3:20 ` [RFC PATCH] " Michel Lespinasse 2012-08-22 3:20 ` Michel Lespinasse 2012-08-22 3:29 ` Rik van Riel 2012-08-22 3:29 ` Rik van Riel 2013-06-03 19:50 ` Daniel Forrest 2013-06-03 19:50 ` Daniel Forrest 2013-06-04 10:37 ` Rik van Riel 2013-06-04 10:37 ` Rik van Riel 2013-06-05 14:02 ` Andrea Arcangeli 2013-06-05 14:02 ` Andrea Arcangeli 2014-11-14 16:30 ` [PATCH] " Daniel Forrest 2014-11-14 16:30 ` Daniel Forrest 2014-11-18 0:02 ` Andrew Morton 2014-11-18 0:02 ` Andrew Morton 2014-11-18 1:41 ` Daniel Forrest [this message] 2014-11-18 1:41 ` Daniel Forrest 2014-11-18 2:41 ` Rik van Riel 2014-11-18 2:41 ` Rik van Riel 2014-11-18 20:19 ` Andrew Morton 2014-11-18 20:19 ` Andrew Morton 2014-11-18 22:15 ` Konstantin Khlebnikov 2014-11-18 22:15 ` Konstantin Khlebnikov 2014-11-18 23:02 ` Konstantin Khlebnikov 2014-11-18 23:50 ` Vlastimil Babka 2014-11-18 23:50 ` Vlastimil Babka 2014-11-19 14:36 ` Konstantin Khlebnikov 2014-11-19 14:36 ` Konstantin Khlebnikov 2014-11-19 16:09 ` Vlastimil Babka 2014-11-19 16:09 ` Vlastimil Babka 2014-11-19 16:58 ` Konstantin Khlebnikov 2014-11-19 16:58 ` Konstantin Khlebnikov 2014-11-19 23:14 ` Michel Lespinasse 2014-11-19 23:14 ` Michel Lespinasse 2014-11-20 14:42 ` Konstantin Khlebnikov 2014-11-20 14:42 ` Konstantin Khlebnikov 2014-11-20 14:50 ` Rik van Riel 2014-11-20 14:50 ` Rik van Riel 2014-11-20 15:03 ` Konstantin Khlebnikov 2014-11-20 15:03 ` Konstantin Khlebnikov 2014-11-24 7:09 ` Konstantin Khlebnikov 2014-11-25 10:59 ` Michal Hocko 2014-11-25 10:59 ` Michal Hocko 2014-11-25 12:13 ` Konstantin Khlebnikov 2014-11-25 15:00 ` Michal Hocko 2014-11-25 15:00 ` Michal Hocko 2014-11-26 17:35 ` Michal Hocko 2014-11-26 17:35 ` Michal Hocko 2014-12-05 15:44 ` Jerome Marchand 2014-11-20 15:27 ` Michel Lespinasse 2014-11-20 15:27 ` Michel Lespinasse 2014-11-19 2:48 ` Rik van Riel 2014-11-19 2:48 ` Rik van Riel
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=20141118014135.GA17252@cosmos.ssec.wisc.edu \ --to=dan.forrest@ssec.wisc.edu \ --cc=aarcange@redhat.com \ --cc=akpm@linux-foundation.org \ --cc=hughd@google.com \ --cc=linux-kernel@vger.kernel.org \ --cc=linux-mm@kvack.org \ --cc=mhocko@suse.cz \ --cc=riel@redhat.com \ --cc=tim@edgecast.com \ --cc=walken@google.com \ /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: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is an external index of several public inboxes, see mirroring instructions on how to clone and mirror all data and code used by this external index.