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

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Message-ID: <20201022023352.GC300658@carbon.dhcp.thefacebook.com>
Date:   Wed, 21 Oct 2020 19:33:52 -0700
From:   Roman Gushchin <guro@...com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>, 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 Wed, Oct 21, 2020 at 05:15:53PM -0700, Mike Kravetz wrote:
> 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.

Hi Mike,

> 
> 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?

I actually have suggested it few months ago, but the idea was
opposed by Joonsoo: https://lkml.org/lkml/2020/4/3/12 .

The time of a bitmap operation is definitely not an issue in my context,
but I can't speak for something like an embedded/rt case.

Thanks!