From: Hugh Dickins <hughd@google.com>
To: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>,
LKML <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Hugh Dickins <hughd@google.com>,
Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently
Date: Wed, 22 Sep 2021 16:18:09 -0700 (PDT) [thread overview]
Message-ID: <24224366-293a-879-95db-f69abcb0cb70@google.com> (raw)
In-Reply-To: <YUueOUfoamxOvEyO@t490s>
On Wed, 22 Sep 2021, Peter Xu wrote:
>
> Not installing pmd means uffd-minor can still trap any further faults just like
> before, afaiu.
>
> There's a very trivial detail that the pmd missing case will have a very slight
> code path change when the next page fault happens: in __handle_mm_fault() we'll
> first try to go into create_huge_pmd() once, however since shmem didn't provide
> huge_fault(), we'll go the VM_FAULT_FALLBACK path, and things will go like
> before when faulting on a small pte. The next UFFDIO_CONTINUE will allocate
> that missing pmd again, however it'll install a 4K page only.
I think you're mistaken there.
I can't tell you much about ->huge_fault(), something introduced for
DAX I believe; but shmem has managed pmd mappings without it, since
before ->huge_fault() was ever added.
Look for the call to do_set_pmd() in finish_fault(): I think you'll
find that is the way shmem's huge pmds get in.
Earlier in the thread you suggested "shmem_getpage() only returns
small pages": but it can very well return PageTransCompound pages,
head or tail, which arrive at this do_set_pmd().
Hugh
next prev parent reply other threads:[~2021-09-22 23:18 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-22 17:51 [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently Peter Xu
2021-09-22 18:21 ` David Hildenbrand
2021-09-22 18:58 ` Peter Xu
2021-09-22 19:29 ` Yang Shi
2021-09-22 20:04 ` Peter Xu
2021-09-22 20:23 ` Peter Xu
2021-09-24 10:05 ` David Hildenbrand
2021-09-22 19:33 ` Peter Xu
2021-09-22 20:49 ` Axel Rasmussen
2021-09-22 21:20 ` Peter Xu
2021-09-22 23:18 ` Hugh Dickins [this message]
2021-09-22 23:44 ` Peter Xu
2021-09-23 1:22 ` Hugh Dickins
2021-09-23 2:18 ` Peter Xu
2021-09-23 16:47 ` Axel Rasmussen
2021-09-23 17:53 ` Peter Xu
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