From: Yu Xu <xuyu@linux.alibaba.com>
To: Hugh Dickins <hughd@google.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
akpm@linux-foundation.org, gavin.dg@linux.alibaba.com,
Greg Thelen <gthelen@google.com>, Wei Xu <weixugc@google.com>,
Matthew Wilcox <willy@infradead.org>,
Nicholas Piggin <npiggin@gmail.com>,
Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm, thp: relax migration wait when failed to get tail page
Date: Wed, 2 Jun 2021 21:20:32 +0800 [thread overview]
Message-ID: <b05ab98d-3a0d-ec23-96dd-5c970aa61580@linux.alibaba.com> (raw)
In-Reply-To: <alpine.LSU.2.11.2106010947370.1090@eggly.anvils>
On 6/2/21 12:55 AM, Hugh Dickins wrote:
> On Wed, 2 Jun 2021, Xu Yu wrote:
>
>> We notice that hung task happens in a conner but practical scenario when
>> CONFIG_PREEMPT_NONE is enabled, as follows.
>>
>> Process 0 Process 1 Process 2..Inf
>> split_huge_page_to_list
>> unmap_page
>> split_huge_pmd_address
>> __migration_entry_wait(head)
>> __migration_entry_wait(tail)
>> remap_page (roll back)
>> remove_migration_ptes
>> rmap_walk_anon
>> cond_resched
>>
>> Where __migration_entry_wait(tail) is occurred in kernel space, e.g.,
>> copy_to_user, which will immediately fault again without rescheduling,
>> and thus occupy the cpu fully.
>>
>> When there are too many processes performing __migration_entry_wait on
>> tail page, remap_page will never be done after cond_resched.
>>
>> This relaxes __migration_entry_wait on tail page, thus gives remap_page
>> a chance to complete.
>>
>> Signed-off-by: Gang Deng <gavin.dg@linux.alibaba.com>
>> Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
>
> Well caught: you're absolutely right that there's a bug there.
> But isn't cond_resched() just papering over the real bug, and
> what it should do is a "page = compound_head(page);" before the
> get_page_unless_zero()? How does that work out in your testing?
compound_head works. The patched kernel is alive for hours under
our reproducer, which usually makes the vanilla kernel hung after
tens of minutes at most.
If we use compound_head, the behavior of __migration_entry_wait(tail)
changes from "retry fault" to "prevent THP from being split". Is that
right? Then which is preferred? If it were me, I would prefer "retry
fault".
>
> Hugh
>
>> ---
>> mm/migrate.c | 7 +++++--
>> 1 file changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index b234c3f3acb7..df2dc39fe566 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -301,8 +301,11 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>> * is zero; but we must not call put_and_wait_on_page_locked() without
>> * a ref. Use get_page_unless_zero(), and just fault again if it fails.
>> */
>> - if (!get_page_unless_zero(page))
>> - goto out;
>> + if (!get_page_unless_zero(page)) {
>> + pte_unmap_unlock(ptep, ptl);
>> + cond_resched();
>> + return;
>> + }
>> pte_unmap_unlock(ptep, ptl);
>> put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE);
>> return;
>> --
>> 2.20.1.2432.ga663e714
>>
>>
>>
--
Thanks,
Yu
next prev parent reply other threads:[~2021-06-02 13:20 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-01 16:31 [PATCH] mm, thp: relax migration wait when failed to get tail page Xu Yu
2021-06-01 16:49 ` Matthew Wilcox
2021-06-01 16:55 ` Hugh Dickins
2021-06-01 17:11 ` Matthew Wilcox
2021-06-01 19:10 ` Hugh Dickins
2021-06-01 20:27 ` Matthew Wilcox
2021-06-02 3:27 ` Yu Xu
2021-06-02 11:58 ` Matthew Wilcox
2021-06-02 12:59 ` Yu Xu
2021-06-02 13:20 ` Yu Xu [this message]
2021-06-02 15:57 ` Hugh Dickins
2021-06-07 7:24 ` Yu Xu
2021-06-08 4:44 ` Hugh Dickins
2021-06-08 5:43 ` Yu Xu
2021-06-08 6:53 ` Hugh Dickins
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=b05ab98d-3a0d-ec23-96dd-5c970aa61580@linux.alibaba.com \
--to=xuyu@linux.alibaba.com \
--cc=akpm@linux-foundation.org \
--cc=gavin.dg@linux.alibaba.com \
--cc=gthelen@google.com \
--cc=hughd@google.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=npiggin@gmail.com \
--cc=vbabka@suse.cz \
--cc=weixugc@google.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).