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From:   Zi Yan <ziy@nvidia.com>
To:     Roman Gushchin <guro@fb.com>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        <saberlily.xia@hisilicon.com>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>, <kernel-team@fb.com>
Subject: Re: [PATCH v1 0/2] mm: cma: introduce a non-blocking version of
 cma_release()
Date:   Thu, 22 Oct 2020 20:58:10 -0400
Message-ID: <09C785FF-8FEA-4178-9172-4E41C173E82A@nvidia.com>
In-Reply-To: <20201023004739.GH300658@carbon.dhcp.thefacebook.com>
References: <20201022225308.2927890-1-guro@fb.com>
 <F8551A10-E254-44FC-B28E-9E7F8AC14B57@nvidia.com>
 <20201023004739.GH300658@carbon.dhcp.thefacebook.com>
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On 22 Oct 2020, at 20:47, Roman Gushchin wrote:

> On Thu, Oct 22, 2020 at 07:42:45PM -0400, Zi Yan wrote:
>> On 22 Oct 2020, at 18:53, Roman Gushchin wrote:
>>
>>> This small patchset introduces a non-blocking version of cma_release(=
)
>>> and simplifies the code in hugetlbfs, where previously we had to
>>> temporarily drop hugetlb_lock around the cma_release() call.
>>>
>>> It should help Zi Yan on his work on 1 GB THPs: splitting a gigantic
>>> THP under a memory pressure requires a cma_release() call. If it's
>>
>> Thanks for the patch. But during 1GB THP split, we only clear
>> the bitmaps without releasing the pages. Also in cma_release_nowait(),=

>> the first page in the allocated CMA region is reused to store
>> struct cma_clear_bitmap_work, but the same method cannot be used
>> during THP split, since the first page is still in-use. We might
>> need to allocate some new memory for struct cma_clear_bitmap_work,
>> which might not be successful under memory pressure. Any suggestion
>> on where to store struct cma_clear_bitmap_work when I only want to
>> clear bitmap without releasing the pages?
>
> It means we can't use cma_release() there either, because it does clear=

> individual pages. We need to clear the cma bitmap without touching page=
s.
>
> Can you handle an error there?
>
> If so, we can introduce something like int cma_schedule_bitmap_clearanc=
e(),
> which will allocate a work structure and will be able to return -ENOMEM=

> in the unlikely case of error.
>
> Will it work for you?

Yes, it works. Thanks.

=E2=80=94
Best Regards,
Yan Zi

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