linux-kernel - Re: [PATCH rfc 0/2] mm: cma: make cma

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Date:   Wed, 21 Oct 2020 17:15:53 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Zi Yan <ziy@...dia.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com
Subject: Re: [PATCH rfc 0/2] mm: cma: make cma_release() non-blocking

On 10/16/20 3:52 PM, Roman Gushchin wrote:
> This small patchset makes cma_release() non-blocking 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
> a blocking function, it complicates the already complicated code.
> Because there are at least two use cases like this (hugetlbfs is
> another example), I believe it's just better to make cma_release()
> non-blocking.
> 
> It also makes it more consistent with other memory releasing functions
> in the kernel: most of them are non-blocking.

Thanks for looking into this Roman.

I may be missing something, but why does cma_release have to be blocking
today?  Certainly, it takes the bitmap in cma_clear_bitmap and could
block.  However, I do not see why cma->lock has to be a mutex.  I may be
missing something, but I do not see any code protected by the mutex doing
anything that could sleep?

Could we simply change that mutex to a spinlock?
-- 
Mike Kravetz