On Tue, May 19, 2026 at 07:07:37PM +0800, Barry Song wrote: > On Tue, May 19, 2026 at 5:21 AM Yang Shi <[email protected]> wrote: > > > > On Sun, May 17, 2026 at 1:45 AM Barry Song <[email protected]> wrote: > > > > > > On Sat, May 2, 2026 at 1:58 AM Matthew Wilcox <[email protected]> wrote: > > > > > > > > On Sat, May 02, 2026 at 01:44:34AM +0800, Barry Song wrote: > > > > > On Fri, May 1, 2026 at 10:57 PM Matthew Wilcox <[email protected]> > > > > > wrote: > > > > > > > > > > > > On Fri, May 01, 2026 at 06:49:58AM +0800, Barry Song wrote: > > > > > > > 1. There is no deterministic latency for I/O completion. It > > > > > > > depends on > > > > > > > both the hardware and the software stack (bio/request queues and > > > > > > > the > > > > > > > block scheduler). Sometimes the latency is short; at other times > > > > > > > it can > > > > > > > be quite long. In such cases, a high-priority thread performing > > > > > > > operations > > > > > > > such as mprotect, unmap, prctl_set_vma, or madvise may be forced > > > > > > > to wait > > > > > > > for an unpredictable amount of time. > > > > > > > > > > > > But does that actually happen? I find it hard to believe that > > > > > > thread A > > > > > > unmaps a VMA while thread B is in the middle of taking a page fault > > > > > > in > > > > > > that same VMA. mprotect() and madvise() are more likely to happen, > > > > > > but > > > > > > it still seems really unlikely to me. > > > > > > > > > > It doesn’t have to involve unmapping or applying mprotect to > > > > > the entire VMA—just a portion of it is sufficient. > > > > > > > > Yes, but that still fails to answer "does this actually happen". How > > > > much > > > > performance is all this complexity in the page fault handler buying us? > > > > If you don't answer this question, I'm just going to go in and rip it > > > > all out. > > > > > > > > > > Hi Matthew (and Lorenzo, Jan, and anyone else who may be > > > waiting for answers), > > > > > > As promised during LSF/MM/BPF, we conducted thorough > > > testing on Android phones to determine whether performing > > > I/O in `filemap_fault()` can block `vma_start_write()`. > > > I wanted to give a quick update on this question. > > > > > > Nanzhe at Xiaomi created tracing scripts and ran various > > > applications on Android devices with I/O performed under > > > the VMA lock in `filemap_fault()`. We found that: > > > > > > 1. There are very few cases where unmap() is blocked by > > > page faults. I assume this is due to buggy user code > > > or poor synchronization between reads and unmap(). > > > So I assume it is not a problem. > > > > > > 2. We observed many cases where `vma_start_write()` > > > is blocked by page-fault I/O in some applications. > > > The blocking occurs in the `dup_mmap()` path during > > > fork(). > > > > > > With Suren's commit fb49c455323ff ("fork: lock VMAs of > > > the parent process when forking"), we now always hold > > > `vma_write_lock()` for each VMA. Note that the > > > `mmap_lock` write lock is also held, which could lead to > > > chained waiting if page-fault I/O is performed without > > > releasing the VMA lock. > > > > > > My gut feeling is that Suren's commit may be overshooting, > > > so my rough idea is that we might want to do something like > > > the following (we haven't tested it yet and it might be > > > wrong): > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 2311ae7c2ff4..5ddaf297f31a 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -1762,7 +1762,13 @@ __latent_entropy int dup_mmap(struct mm_struct > > > *mm, struct mm_struct *oldmm) > > > for_each_vma(vmi, mpnt) { > > > struct file *file; > > > > > > - retval = vma_start_write_killable(mpnt); > > > + /* > > > + * For anonymous or writable private VMAs, prevent > > > + * concurrent CoW faults. > > > + */ > > > + if (!mpnt->vm_file || (!(mpnt->vm_flags & VM_SHARED) && > > > + (mpnt->vm_flags & VM_WRITE))) > > > + retval = vma_start_write_killable(mpnt); > > > if (retval < 0) > > > goto loop_out; > > > if (mpnt->vm_flags & VM_DONTCOPY) { > > > > Maybe a little bit off topic. This is an interesting idea. It seems > > possible we don't have to take vma write lock unconditionally. IIUC > > the write lock is mainly used to serialize against page fault and > > madvise, right? I got a crazy idea off the top of my head. We may be > > able to just take vma write lock iff vma->anon_vma is not NULL. > > > > First of all, write mmap_lock is held, so the vma can't go or be > > changed under us. > > > > Secondly, if vma->anon_vma is NULL, it basically means either no page > > fault happened or no cow happened, so there is no page table to copy, > > this is also what copy_page_range() does currently. So we can shrink > > the critical section to: > > > > if (vma->anon_vma) { > > vma_start_write_killable(src_vma); > > anon_vma_fork(dst_vma, src_vma); > > copy_page_range(dst_vma, src_vma); > > } > > > > But page fault can happen before write mmap_lock is taken, when we > > check vma->anon_vma, it is possible it has not been set up yet. But it > > seems to be equivalent to page fault after fork and won't break the > > semantic. > > Re-reading Suren's commit log for fb49c455323ff8 > ("fork: lock VMAs of the parent process when forking"), > it seems that vm_start_write() is used to protect > against a race where anon_vma changes from NULL to > non-NULL during fork. In that scenario, we hold the > mmap_lock write lock, but not vma_start_write(), so a > concurrent anon_vma_prepare() could still install an > anon_vma. > > " A concurrent page fault on a page newly marked read-only by the page > copy might trigger wp_page_copy() and a anon_vma_prepare(vma) on the > source vma, defeating the anon_vma_clone() that wasn't done because the > parent vma originally didn't have an anon_vma, but we now might end up > copying a pte entry for a page that has one. > " > > If that is the case, then your change does not work. > > Nowadays, nobody calls anon_vma_prepare(vma) directly.
I see callers? Am I imagining them? :) https://elixir.bootlin.com/linux/v7.0.9/A/ident/anon_vma_prepare > Instead, vmf_anon_prepare() is used, and we always > require the mmap_lock read lock before calling > __anon_vma_prepare(). As a result, anon_vma cannot > transition from NULL to non-NULL during fork. Right, yes the mmap read lock is required for that. > > So the original race condition has effectively > disappeared. Err the page tables? All the other cases which require page table copying? Concurrent faults mean that copy_page_range() and faulting with vma->anon_vma _or_ any of the multiple cases mentioned elsewhere. And who knows what else serialises on that. > > You also mentioned the madvise() case. If I understand > correctly, madvise() should take mmap_lock before > modifying anon_vma. Only some parts of madvise() can > support per-VMA locking. Therefore, we probably do not > need: > > if (vma->anon_vma) { > vma_start_write_killable(src_vma); > ... > } I like how you hand wave the VMA lock operations in madvise() :) (Maybe) guard regions being present cause page tables to be copied, they're installed under VMA (read) lock, and can race now. And it sets traps for future changes - introducing more horrible edge case race conditions in fork is just a big nope nope nope. This isn't an area to play around in. > > > > > Anyway, just a crazy idea, I may miss some corner cases. > > To me, it seems that we could remove vma_start_write() > entirely now. Or is that an even crazier idea? As above that'd be totally broken. NAK. > > Thanks > Barry Thanks, Lorenzo
