On 11/23/22 10:09, Peter Xu wrote: > On Wed, Nov 23, 2022 at 10:40:40AM +0100, David Hildenbrand wrote: > > Let me try understand the basic problem first: > > > > hugetlb walks page tables semi-lockless: while we hold the mmap lock, we > > don't grab the page table locks. That's very hugetlb specific handling and I > > assume hugetlb uses different mechanisms to sync against MADV_DONTNEED, > > concurrent page fault s... but that's no news. hugetlb is weird in many ways > > :) > > > > So, IIUC, you want a mechanism to synchronize against PMD unsharing. Can't > > we use some very basic locking for that? > > Yes we can in most cases. Please refer to above paragraph [1] where I > referred Mike's recent work on vma lock. That's the basic locking we need > so far to protect pmd unsharing. I'll attach the link too in the next > post, which is here: > > https://lore.kernel.org/r/20220914221810.95771-1-mike.kravetz@xxxxxxxxxx > > > > > Using RCU / disabling local irqs seems a bit excessive because we *are* > > holding the mmap lock and only care about concurrent unsharing > > The series wanted to address where the vma lock is not easy to take. It > originates from when I was reading Mike's other patch, I forgot why I did > that but I just noticed there's some code path that we may not want to take > a sleepable lock, e.g. in follow page code. Yes, it was the patch suggested by David, https://lore.kernel.org/linux-mm/20221030225825.40872-1-mike.kravetz@xxxxxxxxxx/ The issue was that FOLL_NOWAIT could be passed into follow_page_mask. If so, then we do not want potentially sleep on the mutex. Since you both are on this thread, I thought of/noticed a related issue. In follow_hugetlb_page, it looks like we can call hugetlb_fault if FOLL_NOWAIT is set. hugetlb_fault certainly has the potential for sleeping. Is this also a similar issue? -- Mike Kravetz > The other one is huge_pmd_share() where we may have the mmap lock for > current mm but we're fundamentally walking another mm. It'll be tricky to > take a sleepable lock in such condition too. > > I mentioned these cases in the other paragraph above [2]. Let me try to > expand that in my next post too. > > It's debatable whether all the rest places can only work with either RCU or > irq disabled, but the idea is at least it should speed up those paths when > we still can. Here, irqoff might be a bit heavy, but RCU lock should be > always superior to vma lock when possible, the payoff is we may still see > stale pgtable data (since unsharing can still happen in parallel), while > that can be completely avoided when we take the vma lock. > > Thanks, > > -- > Peter Xu >
