From: Suren Baghdasaryan <surenb@google.com>
To: Peter Xu <peterx@redhat.com>
Cc: akpm@linux-foundation.org, willy@infradead.org,
hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com,
jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
michel@lespinasse.org, liam.howlett@oracle.com,
jglisse@google.com, vbabka@suse.cz, minchan@google.com,
dave@stgolabs.net, punit.agrawal@bytedance.com,
lstoakes@gmail.com, hdanton@sina.com, apopple@nvidia.com,
ying.huang@intel.com, david@redhat.com, yuzhao@google.com,
dhowells@redhat.com, hughd@google.com, viro@zeniv.linux.org.uk,
brauner@kernel.org, pasha.tatashin@soleen.com,
linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration
Date: Fri, 9 Jun 2023 18:29:43 -0700 [thread overview]
Message-ID: <CAJuCfpG3PrbGxpDAEkyGQXW88+otb=FsbrhPJ4ePN7Xhn0a+_A@mail.gmail.com> (raw)
In-Reply-To: <CAJuCfpHKUjAwgWbxvJQDyEnneRD03p2M6247Q6=3-oOq_FL7zA@mail.gmail.com>
On Fri, Jun 9, 2023 at 3:30 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Fri, Jun 9, 2023 at 1:42 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote:
> > > migration_entry_wait does not need VMA lock, therefore it can be dropped
> > > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> > > lock was dropped while in handle_mm_fault().
> > > Note that once VMA lock is dropped, the VMA reference can't be used as
> > > there are no guarantees it was not freed.
> >
> > Then vma lock behaves differently from mmap read lock, am I right? Can we
> > still make them match on behaviors, or there's reason not to do so?
>
> I think we could match their behavior by also dropping mmap_lock here
> when fault is handled under mmap_lock (!(fault->flags &
> FAULT_FLAG_VMA_LOCK)).
> I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping
> mmap_lock in do_page_fault(), so indeed, I might be able to use
> VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well
> instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea
> of reusing existing flags?
Sorry, I meant VM_FAULT_VMA_UNLOCKED, not FAULT_FLAG_VMA_LOCK in the
above reply.
I took a closer look into using VM_FAULT_COMPLETED instead of
VM_FAULT_VMA_UNLOCKED but when we fall back from per-vma lock to
mmap_lock we need to retry with an indication that the per-vma lock
was dropped. Returning (VM_FAULT_RETRY | VM_FAULT_COMPLETE) to
indicate such state seems strange to me ("retry" and "complete" seem
like contradicting concepts to be used in a single result). I could
use VM_FAULT_COMPLETE when releasing mmap_lock since we don't use it
in combination with VM_FAULT_RETRY and (VM_FAULT_RETRY |
VM_FAULT_VMA_UNLOCKED) when dropping per-vma lock and falling back to
mmap_lock. It still requires the new VM_FAULT_VMA_UNLOCKED flag but I
think logically that makes more sense. WDYT?
>
> >
> > One reason is if they match they can reuse existing flags and there'll be
> > less confusing, e.g. this:
> >
> > (fault->flags & FAULT_FLAG_VMA_LOCK) &&
> > (vm_fault_ret && (VM_FAULT_RETRY || VM_FAULT_COMPLETE))
> >
> > can replace the new flag, iiuc.
> >
> > Thanks,
> >
> > --
> > Peter Xu
> >
> > --
> > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@android.com.
> >
next prev parent reply other threads:[~2023-06-10 1:30 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-09 0:51 [PATCH v2 0/6] Per-vma lock support for swap and userfaults Suren Baghdasaryan
2023-06-09 0:51 ` [PATCH v2 1/6] swap: remove remnants of polling from read_swap_cache_async Suren Baghdasaryan
2023-06-09 1:57 ` Huang, Ying
2023-06-09 3:13 ` Ming Lei
2023-06-09 18:50 ` Suren Baghdasaryan
2023-06-12 4:53 ` Christoph Hellwig
2023-06-09 0:51 ` [PATCH v2 2/6] mm: handle swap page faults under VMA lock if page is uncontended Suren Baghdasaryan
2023-06-09 20:25 ` Peter Xu
2023-06-09 20:35 ` Matthew Wilcox
2023-06-09 20:45 ` Peter Xu
2023-06-09 22:34 ` Suren Baghdasaryan
2023-06-12 13:59 ` Peter Xu
2023-06-12 16:09 ` Suren Baghdasaryan
2023-06-09 0:51 ` [PATCH v2 3/6] mm: add missing VM_FAULT_RESULT_TRACE name for VM_FAULT_COMPLETED Suren Baghdasaryan
2023-06-09 20:34 ` Peter Xu
2023-06-09 0:51 ` [PATCH v2 4/6] mm: drop VMA lock before waiting for migration Suren Baghdasaryan
2023-06-09 20:42 ` Peter Xu
2023-06-09 22:30 ` Suren Baghdasaryan
2023-06-10 1:29 ` Suren Baghdasaryan [this message]
2023-06-12 13:36 ` Peter Xu
2023-06-12 16:07 ` Suren Baghdasaryan
2023-06-12 18:34 ` Peter Xu
2023-06-12 18:44 ` Suren Baghdasaryan
2023-06-12 18:57 ` Peter Xu
2023-06-12 13:28 ` Peter Xu
2023-06-12 15:41 ` Suren Baghdasaryan
2023-06-09 0:51 ` [PATCH v2 5/6] mm: implement folio wait under VMA lock Suren Baghdasaryan
2023-06-09 15:03 ` Matthew Wilcox
2023-06-09 18:49 ` Suren Baghdasaryan
2023-06-09 18:55 ` Suren Baghdasaryan
2023-06-09 19:36 ` Matthew Wilcox
2023-06-09 22:49 ` Suren Baghdasaryan
2023-06-09 20:54 ` Peter Xu
2023-06-09 22:48 ` Suren Baghdasaryan
2023-06-12 13:56 ` Peter Xu
2023-06-09 0:51 ` [PATCH v2 6/6] mm: handle userfaults " Suren Baghdasaryan
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