linux-kernel - Re: [PATCH v1 0/2] mm: cma: introduce a non-blocking version of cma

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Date:   Thu, 22 Oct 2020 20:58:10 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Roman Gushchin <guro@...com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        <saberlily.xia@...ilicon.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <kernel-team@...com>
Subject: Re: [PATCH v1 0/2] mm: cma: introduce a non-blocking version of
 cma_release()

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 pages.
>
> Can you handle an error there?
>
> If so, we can introduce something like int cma_schedule_bitmap_clearance(),
> 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.

—
Best Regards,
Yan Zi

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