On Fri, Oct 21, 2011 at 11:56 PM, Mel Gorman <mgorman@xxxxxxx> wrote: > On Thu, Oct 20, 2011 at 05:11:28PM +0800, Nai Xia wrote: >> On Mon, Oct 17, 2011 at 7:54 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote: >> > On Thu, Oct 13, 2011 at 04:30:09PM -0700, Hugh Dickins wrote: >> >> mremap's down_write of mmap_sem, together with i_mmap_mutex/lock, >> >> and pagetable locks, were good enough before page migration (with its >> >> requirement that every migration entry be found) came in; and enough >> >> while migration always held mmap_sem. But not enough nowadays, when >> >> there's memory hotremove and compaction: anon_vma lock is also needed, >> >> to make sure a migration entry is not dodging around behind our back. >> > >> > For things like migrate and split_huge_page, the anon_vma layer must >> > guarantee the page is reachable by rmap walk at all times regardless >> > if it's at the old or new address. >> > >> > This shall be guaranteed by the copy_vma called by move_vma well >> > before move_page_tables/move_ptes can run. >> > >> > copy_vma obviously takes the anon_vma lock to insert the new "dst" vma >> > into the anon_vma chains structures (vma_link does that). That before >> > any pte can be moved. >> > >> > Because we keep two vmas mapped on both src and dst range, with >> > different vma->vm_pgoff that is valid for the page (the page doesn't >> > change its page->index) the page should always find _all_ its pte at >> > any given time. >> > >> > There may be other variables at play like the order of insertion in >> > the anon_vma chain matches our direction of copy and removal of the >> > old pte. But I think the double locking of the PT lock should make the >> > order in the anon_vma chain absolutely irrelevant (the rmap_walk >> > obviously takes the PT lock too), and furthermore likely the >> > anon_vma_chain insertion is favorable (the dst vma is inserted last >> > and checked last). But it shouldn't matter. >> >> I happened to be reading these code last week. >> >> And I do think this order matters, the reason is just quite similar why we >> need i_mmap_lock in move_ptes(): >> If rmap_walk goes dst--->src, then when it first look into dst, ok, the > > You might be right in that the ordering matters. We do link new VMAs at > the end of the list in anon_vma_chain_list so remove_migrate_ptes should > be walking from src->dst. > > If remove_migrate_pte finds src first, it will remove the pte and the > correct version will get copied. If move_ptes runs between when > remove_migrate_ptes moves from src to dst, then the PTE at dst will > still be correct. > >> pte is not there, and it happily skip it and release the PTL. >> Then just before it look into src, move_ptes() comes in, takes the locks >> and moves the pte from src to dst. And then when rmap_walk() look >> into src, it will find an empty pte again. The pte is still there, >> but rmap_walk() missed it ! >> > > I believe the ordering is correct though and protects us in this case. > >> IMO, this can really happen in case of vma_merge() succeeding. >> Imagine that src vma is lately faulted and in anon_vma_prepare() >> it got a same anon_vma with an existing vma ( named evil_vma )through >> find_mergeable_anon_vma(). This can potentially make the vma_merge() in >> copy_vma() return with evil_vma on some new relocation request. But src_vma >> is really linked _after_ evil_vma/new_vma/dst_vma. >> In this way, the ordering protocol of anon_vma chain is broken. >> This should be a rare case because I think in most cases >> if two VMAs can reusable_anon_vma() they were already merged. >> >> How do you think ? >> > > Despite the comments in anon_vma_compatible(), I would expect that VMAs > that can share an anon_vma from find_mergeable_anon_vma() will also get > merged. When the new VMA is created, it will be linked in the usual > manner and the oldest->newest ordering is what is required. That's not > that important though. > > What is important is if mremap is moving src to a dst that is adjacent > to another anon_vma. If src has never been faulted, it's not an issue > because there are also no migration PTEs. If src has been faulted, then > is_mergeable_anon_vma() should fail as anon_vma1 != anon_vma2 and they > are not compatible. The ordering is preserved and we are still ok. Hi Mel, Thanks for input. I agree on _almost_ all your reasoning above. But there is a tricky series of events I mentioned in https://lkml.org/lkml/2011/10/21/14 , which, I think, can really lead to anon_vma1 == anon_vma2 in this case. These events is led by a failure when do_brk() fails on vma_merge() due to ENOMEM, rare it maybe though, And I am still not sure if there exists any other corner cases when a "should be merged" VMAs just sit there side by side for sth reason -- normally, that does not trigger BUGs, so maybe hard to detect in real workload. Please refer to my link and I think the construction was very clear if I had not missed sth subtle. Thanks, Nai Xia > > All that said, while I don't think there is a problem, I can't convince > myself 100% of it. Andrea, can you spot a flaw? > > -- > Mel Gorman > SUSE Labs > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href