From: Mike Kravetz <mike.kravetz@oracle.com>
To: Minchan Kim <minchan@kernel.org>, David Hildenbrand <david@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
Andrew Morton <akpm@linux-foundation.org>,
syzbot <syzbot+acf65ca584991f3cc447@syzkaller.appspotmail.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
llvm@lists.linux.dev, nathan@kernel.org, ndesaulniers@google.com,
syzkaller-bugs@googlegroups.com, trix@redhat.com,
Matthew Wilcox <willy@infradead.org>,
Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [syzbot] WARNING in follow_hugetlb_page
Date: Sat, 21 May 2022 16:50:50 -0700 [thread overview]
Message-ID: <6daa971b-03f1-0f69-4c85-ae4796d339e0@oracle.com> (raw)
In-Reply-To: <YokufHxnkHUk5UR6@google.com>
On 5/21/22 11:25, Minchan Kim wrote:
> On Sat, May 21, 2022 at 06:46:27PM +0200, David Hildenbrand wrote:
>
> < snip >
>
>>>> The general rules are:
>>>>
>>>> ZONE_MOVABLE: nobody is allowed to place unmovable allocations there; it
>>>> could prevent memory offlining/unplug.
>>>>
>>>> CMA: nobody *but the designated owner* is allowed to place unmovable
>>>> memory there; it could prevent the actual owner to allocate contiguous
>>>> memory.
>>>
>>> I am confused what's the meaning of designated owner and actuall owner
>>> in your context.
>>
>> designated==actual here. I just wanted to distinguish from someone
>> current temporary owner of the page ("allocated it via a movable
>> allocation") but the actual designated owner (e.g., hugetlb CMA)
>>
>> The page/memory owner terminology is just confusing. Let's rephrase to:
>> only the CMA area owner is allowed to place unmovable allocations there.
>
> Yeah, the CMA area owner is much better.
>
>>
>>>
>>> What I thought about the issue based on you explanation:
>>>
>>> HugeTLB allocates its page by two types of allocation
>>>
>>> 1. alloc_pages(GFP_MOVABLE)
>>>
>>> It could allocate the hugetlb page from CMA area but longterm pin
>>> should migrate them out of cma before the pinning so allowing
>>> the pinning on the page is no problem and current code works like
>>> that.
>>>
>>> check_and_migrate_movable_pages
>>>
>>
>> Yes.
>>
>>> 2. cma_alloc
>>>
>>> The cma_alloc is used only for *gigantic page* and the hugetlbfs
>>> is the very owner of the page. IOW, if the hugetlbfs was succeeded
>>> to allocate the gigantic page by cma_alloc, there is no other
>>> owner to be able to claim the page any longer so it's fine to
>>> allow longterm pinning againt the gingantic page but current.
>>> However, current code doesn't work like that due to
>>> is_pinnable_page. IOW, hugetlbfs need a way to distinguish
>>> whether the page owner is hugetlbfs or not.
>>>
>>> Are we on same page?
>>
>> Yes, exactly. What I wanted to express is: for huge pages we have to
>> make a smarter decision because there are cases where we want to
>> migrate, and cases where we don't want to migrate.
>
> Sure, maybe hugetlbfs could squeeze a bit in one of subpage of the
> CMA compound page. "I am CMA allocated but allow to pinned for longterm"
>
Thanks for all the ideas here. Yes, we already have a whole word for hugetlb
specific page flags (see hugetlb_page_flags in linux/hugetlb.h). I'm pretty
sure I even proposed a 'allocated from CMA' flag, but there was another way to
get that information. We can add such a flag to for the purpose if making a
decision about long term pinning.
BTW - It is possible that a gigantic page allocated in CMA could be demoted
(split) into smaller hugetlb pages. I 'think' we would also want to allow
long term pinning in this case.
--
Mike Kravetz
next prev parent reply other threads:[~2022-05-21 23:51 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-13 9:03 [syzbot] WARNING in follow_hugetlb_page syzbot
2022-05-13 16:43 ` syzbot
2022-05-13 17:26 ` Andrew Morton
2022-05-13 18:09 ` Mike Kravetz
2022-05-13 22:48 ` Mike Kravetz
2022-05-13 23:19 ` Andrew Morton
2022-05-13 23:54 ` Minchan Kim
2022-05-14 0:09 ` John Hubbard
2022-05-14 0:26 ` Minchan Kim
2022-05-14 0:56 ` John Hubbard
2022-05-14 1:16 ` John Hubbard
2022-05-17 3:37 ` Mike Kravetz
2022-05-18 7:12 ` John Hubbard
2022-05-20 22:19 ` Minchan Kim
2022-05-20 22:56 ` John Hubbard
2022-05-20 23:25 ` Minchan Kim
2022-05-20 23:31 ` Mike Kravetz
2022-05-20 23:43 ` Minchan Kim
2022-05-21 0:04 ` Mike Kravetz
2022-05-21 15:24 ` Minchan Kim
2022-05-21 15:51 ` David Hildenbrand
2022-05-21 16:36 ` Minchan Kim
2022-05-21 16:46 ` David Hildenbrand
2022-05-21 18:25 ` Minchan Kim
2022-05-21 23:50 ` Mike Kravetz [this message]
2022-05-14 0:18 ` Andrew Morton
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=6daa971b-03f1-0f69-4c85-ae4796d339e0@oracle.com \
--to=mike.kravetz@oracle.com \
--cc=akpm@linux-foundation.org \
--cc=david@redhat.com \
--cc=jhubbard@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=llvm@lists.linux.dev \
--cc=minchan@kernel.org \
--cc=nathan@kernel.org \
--cc=ndesaulniers@google.com \
--cc=sfr@canb.auug.org.au \
--cc=syzbot+acf65ca584991f3cc447@syzkaller.appspotmail.com \
--cc=syzkaller-bugs@googlegroups.com \
--cc=trix@redhat.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).