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Date:   Fri, 23 Oct 2020 13:55:01 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Zi Yan <ziy@nvidia.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()
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On Thu, Oct 22, 2020 at 08:58:10PM -0400, Zi Yan wrote:
> 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.

Nice!

It means we can keep these two patches as they are (they do makes sense
even without THP, because they simplify the hugetlbfs code).

I'll send a patch implementing cma_schedule_bitmap_clearance() to you,
so you'll be able to use it in the patchset.

Thanks!