From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: Jan Kara <jack@suse.cz>
Cc: Peter Xu <peterx@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Maya B . Gokhale" <gokhale2@llnl.gov>,
Linus Torvalds <torvalds@linux-foundation.org>,
Yang Shi <yang.shi@linux.alibaba.com>,
Marty Mcfadden <mcfadden8@llnl.gov>,
Kirill Shutemov <kirill@shutemov.name>,
Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Mon, 24 Aug 2020 18:37:53 +0300 [thread overview]
Message-ID: <dd6eb3e6-2797-1cf3-e1af-62a809ce83f2@virtuozzo.com> (raw)
In-Reply-To: <20200824143010.GG24877@quack2.suse.cz>
On 24.08.2020 17:30, Jan Kara wrote:
> On Mon 24-08-20 11:36:22, Kirill Tkhai wrote:
>> On 22.08.2020 02:49, Peter Xu wrote:
>>> From: Linus Torvalds <torvalds@linux-foundation.org>
>>>
>>> How about we just make sure we're the only possible valid user fo the
>>> page before we bother to reuse it?
>>>
>>> Simplify, simplify, simplify.
>>>
>>> And get rid of the nasty serialization on the page lock at the same time.
>>>
>>> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
>>> [peterx: add subject prefix]
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>> mm/memory.c | 59 +++++++++++++++--------------------------------------
>>> 1 file changed, 17 insertions(+), 42 deletions(-)
>>>
>>> diff --git a/mm/memory.c b/mm/memory.c
>>> index 602f4283122f..cb9006189d22 100644
>>> --- a/mm/memory.c
>>> +++ b/mm/memory.c
>>> @@ -2927,50 +2927,25 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>>> * not dirty accountable.
>>> */
>>> if (PageAnon(vmf->page)) {
>>> - int total_map_swapcount;
>>> - if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
>>> - page_count(vmf->page) != 1))
>>> + struct page *page = vmf->page;
>>> +
>>> + /* PageKsm() doesn't necessarily raise the page refcount */
>>
>> No, this is wrong. PageKSM() always raises refcount.
>
> OK, then I'm confused. The comment before get_ksm_page() states:
>
> * get_ksm_page: checks if the page indicated by the stable node
> * is still its ksm page, despite having held no reference to it.
> * In which case we can trust the content of the page, and it
> * returns the gotten page; but if the page has now been zapped,
> * remove the stale node from the stable tree and return NULL.
> ...
> * You would expect the stable_node to hold a reference to the ksm page.
> * But if it increments the page's count, swapping out has to wait for
> * ksmd to come around again before it can free the page, which may take
> * seconds or even minutes: much too unresponsive. So instead we use a
> * "keyhole reference": access to the ksm page from the stable node peeps
> * out through its keyhole to see if that page still holds the right key,
> * pointing back to this stable node.
>
> So this all seems to indicate that KSM doesn't hold a proper page reference
> and relies on anyone making page writeable to change page->mapping so that
> KSM notices this and doesn't use the page anymore... Am I missing
> something?
Sure, KSM does not increment page counter, when a page becomes PageKsm().
Is patch comment about that? Even if so, I don't understand what this
comment is about. "PageKsm() does not take additional counter" is not
a reason the page can't be reused there. The reason is that readers
of this page may increase a counter without taking the lock, so
this page_count() == 1 under the lock does not guarantee anything.
>> There was another
>> problem: KSM may raise refcount without lock_page(), and only then it
>> takes the lock. See get_ksm_page(GET_KSM_PAGE_NOLOCK) for the details.
>>
>> So, reliable protection against parallel access requires to freeze page
>> counter, which is made in reuse_ksm_page().
>
> OK, this as well.
>
> Honza
>
>>
>>> + if (PageKsm(page) || page_count(page) != 1)
>>> + goto copy;
>>> + if (!trylock_page(page))
>>> + goto copy;
>>> + if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
>>> + unlock_page(page);
>>> goto copy;
>>> - if (!trylock_page(vmf->page)) {
>>> - get_page(vmf->page);
>>> - pte_unmap_unlock(vmf->pte, vmf->ptl);
>>> - lock_page(vmf->page);
>>> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>>> - vmf->address, &vmf->ptl);
>>> - if (!pte_same(*vmf->pte, vmf->orig_pte)) {
>>> - update_mmu_tlb(vma, vmf->address, vmf->pte);
>>> - unlock_page(vmf->page);
>>> - pte_unmap_unlock(vmf->pte, vmf->ptl);
>>> - put_page(vmf->page);
>>> - return 0;
>>> - }
>>> - put_page(vmf->page);
>>> - }
>>> - if (PageKsm(vmf->page)) {
>>> - bool reused = reuse_ksm_page(vmf->page, vmf->vma,
>>> - vmf->address);
>>> - unlock_page(vmf->page);
>>> - if (!reused)
>>> - goto copy;
>>> - wp_page_reuse(vmf);
>>> - return VM_FAULT_WRITE;
>>> - }
>>> - if (reuse_swap_page(vmf->page, &total_map_swapcount)) {
>>> - if (total_map_swapcount == 1) {
>>> - /*
>>> - * The page is all ours. Move it to
>>> - * our anon_vma so the rmap code will
>>> - * not search our parent or siblings.
>>> - * Protected against the rmap code by
>>> - * the page lock.
>>> - */
>>> - page_move_anon_rmap(vmf->page, vma);
>>> - }
>>> - unlock_page(vmf->page);
>>> - wp_page_reuse(vmf);
>>> - return VM_FAULT_WRITE;
>>> }
>>> - unlock_page(vmf->page);
>>> + /*
>>> + * Ok, we've got the only map reference, and the only
>>> + * page count reference, and the page is locked,
>>> + * it's dark out, and we're wearing sunglasses. Hit it.
>>> + */
>>> + wp_page_reuse(vmf);
>>> + unlock_page(page);
>>> + return VM_FAULT_WRITE;
>>> } else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
>>> (VM_WRITE|VM_SHARED))) {
>>> return wp_page_shared(vmf);
>>>
>>
next prev parent reply other threads:[~2020-08-24 15:43 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24 8:36 ` Kirill Tkhai
2020-08-24 14:30 ` Jan Kara
2020-08-24 15:37 ` Kirill Tkhai [this message]
2020-08-24 18:22 ` Linus Torvalds
2020-09-01 7:01 ` Hugh Dickins
2020-09-14 14:38 ` Jason Gunthorpe
2020-09-14 17:32 ` Linus Torvalds
2020-09-14 18:34 ` Peter Xu
2020-09-14 21:15 ` Peter Xu
2020-09-14 22:55 ` Jason Gunthorpe
2020-09-14 22:59 ` Linus Torvalds
2020-09-14 23:28 ` Jason Gunthorpe
2020-09-15 0:19 ` Linus Torvalds
2020-09-15 14:50 ` Peter Xu
2020-09-15 15:17 ` Peter Xu
2020-09-15 16:05 ` Jason Gunthorpe
2020-09-15 18:29 ` Jason Gunthorpe
2020-09-15 19:13 ` Peter Xu
2020-09-15 19:38 ` Jason Gunthorpe
2020-09-15 21:33 ` Peter Xu
2020-09-15 23:22 ` Jason Gunthorpe
2020-09-16 1:50 ` John Hubbard
2020-09-16 17:48 ` Jason Gunthorpe
2020-09-16 18:46 ` Peter Xu
2020-09-17 11:25 ` Jason Gunthorpe
2020-09-17 18:11 ` Linus Torvalds
2020-09-17 19:38 ` Jason Gunthorpe
2020-09-17 19:51 ` Linus Torvalds
2020-09-18 16:40 ` Peter Xu
2020-09-18 17:16 ` Linus Torvalds
2020-09-18 19:57 ` Peter Xu
2020-09-18 17:32 ` Jason Gunthorpe
2020-09-18 20:40 ` Peter Xu
2020-09-18 20:59 ` Linus Torvalds
2020-09-19 0:28 ` Jason Gunthorpe
2020-09-18 21:06 ` John Hubbard
2020-09-19 0:01 ` Jason Gunthorpe
2020-09-21 8:35 ` Jan Kara
2020-09-21 12:03 ` Jason Gunthorpe
2022-02-16 16:59 ` Oded Gabbay
2022-02-16 17:24 ` Oded Gabbay
2022-02-16 19:04 ` Linus Torvalds
2022-02-16 19:20 ` Oded Gabbay
2022-02-16 19:24 ` David Hildenbrand
2020-09-21 13:42 ` Michal Hocko
2020-09-21 14:18 ` Peter Xu
2020-09-21 14:28 ` Michal Hocko
2020-09-21 14:38 ` Tejun Heo
2020-09-21 14:43 ` Christian Brauner
2020-09-21 14:55 ` Michal Hocko
2020-09-21 15:04 ` Christian Brauner
2020-09-21 16:06 ` Michal Hocko
2020-09-23 7:53 ` Michal Hocko
2020-09-21 14:41 ` Christian Brauner
2020-09-21 14:57 ` Michal Hocko
2020-09-21 16:31 ` Peter Xu
2020-09-17 18:14 ` Peter Xu
2020-09-17 18:26 ` Linus Torvalds
2020-09-17 19:03 ` Peter Xu
2020-09-17 19:42 ` Linus Torvalds
2020-09-17 19:55 ` John Hubbard
2020-09-17 20:06 ` Jason Gunthorpe
2020-09-17 20:19 ` John Hubbard
2020-09-17 20:25 ` Jason Gunthorpe
2020-09-17 20:35 ` Linus Torvalds
2020-09-17 21:40 ` Peter Xu
2020-09-17 22:09 ` Jason Gunthorpe
2020-09-17 22:25 ` Linus Torvalds
2020-09-17 22:48 ` Ira Weiny
2020-09-18 9:36 ` Jan Kara
2020-09-18 9:44 ` Jan Kara
2020-09-18 16:19 ` Jason Gunthorpe
2020-09-15 10:23 ` Leon Romanovsky
2020-09-15 15:56 ` Jason Gunthorpe
2020-09-15 15:03 ` Oleg Nesterov
2020-09-15 16:18 ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27 ` Oleg Nesterov
2020-09-14 17:59 ` Peter Xu
2020-09-14 19:03 ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14 ` Linus Torvalds
2020-08-24 0:24 ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58 ` Peter Xu
2020-08-24 8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44 ` Jason Gunthorpe
2021-02-02 17:05 ` Peter Xu
2021-02-02 17:13 ` Jason Gunthorpe
2021-02-03 12:43 ` Gal Pressman
2021-02-03 14:00 ` Jason Gunthorpe
2021-02-03 14:47 ` Gal Pressman
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