From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Muchun Song <songmuchun@bytedance.com>,
Peter Feiner <pfeiner@google.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings
Date: Fri, 5 Aug 2022 20:20:52 +0200 [thread overview]
Message-ID: <4f644ac5-c40b-32d4-3234-c1dac3d09f83@redhat.com> (raw)
In-Reply-To: <Yu1dkZeBwv0SUQMq@xz-m1.local>
On 05.08.22 20:12, Peter Xu wrote:
> On Fri, Aug 05, 2022 at 01:03:29PM +0200, David Hildenbrand wrote:
>> Let's add a safety net if we ever get (again) a write-fault on a R/O-mapped
>> page in a shared mapping, in which case we simply have to map the
>> page writable.
>>
>> VM_MAYSHARE handling in hugetlb_fault() for FAULT_FLAG_WRITE
>> indicates that this was at least envisioned, but could never have worked
>> as expected. This theoretically paves the way for softdirty tracking
>> support in hugetlb.
>>
>> Tested without the fix for softdirty tracking.
>>
>> Note that there is no need to do any kind of reservation in hugetlb_fault()
>> in this case ... because we already have a hugetlb page mapped R/O
>> that we will simply map writable and we are not dealing with COW/unsharing.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> mm/hugetlb.c | 21 ++++++++++++++-------
>> 1 file changed, 14 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index a18c071c294e..bbab7aa9d8f8 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5233,6 +5233,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>> VM_BUG_ON(unshare && (flags & FOLL_WRITE));
>> VM_BUG_ON(!unshare && !(flags & FOLL_WRITE));
>>
>> + /* Let's take out shared mappings first, this should be a rare event. */
>> + if (unlikely(vma->vm_flags & VM_MAYSHARE)) {
>
> Should we check VM_SHARED instead?
Relying on VM_SHARED to detect MAP_PRIVATE vs. MAP_SHARED is
unfortunately wrong.
If you're curious, take a look at f83a275dbc5c ("mm: account for
MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs")
and mmap() code.
Long story short: if the file is read-only, we only have VM_MAYSHARE but
not VM_SHARED (and consequently also not VM_MAYWRITE).
>
>> + if (unshare)
>> + return 0;
>
> Curious when will this happen especially if we switch to VM_SHARED above.
> Shouldn't "unshare" not happen at all on a shared region?
FAULT_FLAG_UNSHARE is documented to behave like:
"FAULT_FLAG_UNSHARE is ignored and treated like an ordinary read fault
when no existing R/O-mapped anonymous page is encountered."
It should currently not happen. Focus on should ;)
>
>> + if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>> + return VM_FAULT_SIGSEGV;
>
> I had a feeling that you just want to double check we have write
> permission, but IIUC this should be checked far earlier or we'll have
> problem. No strong opinion if so, but I'd suggest dropping this one,
> otherwise we could add tons of WARN_ON_ONCE() in anywhere in the page fault
> stack and they mostly won't trigger at all.
Not quite. We usually (!hugetlb) have maybe_mkwrite() all over the
place. This is just an indication that we don't have maybe semantics
here. But as we also don't have it for hugetlb anon code below, maybe I
can just drop it. (or check it for both call paths)
>
>> + set_huge_ptep_writable(vma, haddr, ptep);
>
> Do we wanna set dirty bits too?
set_huge_ptep_writable() handles that.
Thanks!
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2022-08-05 18:21 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-05 11:03 [PATCH v1 0/2] mm/hugetlb: fix write-fault handling for shared mappings David Hildenbrand
2022-08-05 11:03 ` [PATCH v1 1/2] mm/hugetlb: fix hugetlb not supporting write-notify David Hildenbrand
2022-08-05 18:14 ` Peter Xu
2022-08-05 18:22 ` David Hildenbrand
2022-08-05 18:23 ` Mike Kravetz
2022-08-05 18:25 ` David Hildenbrand
2022-08-05 18:33 ` Mike Kravetz
2022-08-05 18:57 ` David Hildenbrand
2022-08-05 20:48 ` Mike Kravetz
2022-08-05 23:13 ` Peter Xu
2022-08-05 23:33 ` Mike Kravetz
2022-08-08 16:10 ` Peter Xu
2022-08-08 16:36 ` David Hildenbrand
2022-08-08 19:28 ` Peter Xu
2022-08-10 9:29 ` David Hildenbrand
2022-08-05 11:03 ` [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings David Hildenbrand
2022-08-05 18:12 ` Peter Xu
2022-08-05 18:20 ` David Hildenbrand [this message]
2022-08-08 16:05 ` Peter Xu
2022-08-08 16:25 ` David Hildenbrand
2022-08-08 20:21 ` Peter Xu
2022-08-08 22:08 ` Peter Xu
2022-08-10 9:37 ` David Hildenbrand
2022-08-10 9:45 ` David Hildenbrand
2022-08-10 19:29 ` Peter Xu
2022-08-10 19:40 ` David Hildenbrand
2022-08-10 19:52 ` Peter Xu
2022-08-10 23:55 ` Mike Kravetz
2022-08-11 8:48 ` David Hildenbrand
2022-08-05 23:08 ` Mike Kravetz
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