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Date:   Thu, 3 Mar 2022 17:25:15 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Nadav Amit <nadav.amit@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: madvise: MADV_DONTNEED_LOCKED
Message-ID: <YiFASwUhc1Mz1Jz2@cmpxchg.org>
References: <20220303212956.229409-1-hannes@cmpxchg.org>
 <A5A8E655-0CE6-4F27-85FD-E99726776EAC@gmail.com>
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On Thu, Mar 03, 2022 at 01:35:56PM -0800, Nadav Amit wrote:
> 
> 
> > On Mar 3, 2022, at 1:29 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> > 
> > MADV_DONTNEED historically rejects mlocked ranges, but with
> > MLOCK_ONFAULT and MCL_ONFAULT allowing to mlock without populating,
> > there are valid use cases for depopulating locked ranges as well.
> 
> ...
> 
> > @@ -850,7 +858,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> > 		VM_WARN_ON(start >= end);
> > 	}
> > 
> > -	if (behavior == MADV_DONTNEED)
> > +	if (behavior == MADV_DONTNEED || behavior == MADV_DONTNEED_LOCKED)
> > 		return madvise_dontneed_single_vma(vma, start, end);
> > 	else if (behavior == MADV_FREE)
> > 		return madvise_free_single_vma(vma, start, end);
> > @@ -988,6 +996,7 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
> > 		return madvise_pageout(vma, prev, start, end);
> > 	case MADV_FREE:
> > 	case MADV_DONTNEED:
> > +	case MADV_DONTNEED_LOCKED:
> > 		return madvise_dontneed_free(vma, prev, start, end, behavior);
> > 	case MADV_POPULATE_READ:
> > 	case MADV_POPULATE_WRITE:
> > @@ -1113,6 +1122,7 @@ madvise_behavior_valid(int behavior)
> > 	case MADV_REMOVE:
> > 	case MADV_WILLNEED:
> > 	case MADV_DONTNEED:
> > +	case MADV_DONTNEED_LOCKED:
> > 	case MADV_FREE:
> > 	case MADV_COLD:
> > 	case MADV_PAGEOUT:
> 
> Don’t you want to change madvise_need_mmap_write() as well and add
> MADV_DONTNEED_LOCKED there too?

You're absolutely right, thanks Nadav! It'd be fine, but more
expensive than necessary. Here is the fixlet:

diff --git a/mm/madvise.c b/mm/madvise.c
index 12dfa14bc985..7dbfcd6c955a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -52,6 +52,7 @@ static int madvise_need_mmap_write(int behavior)
 	case MADV_REMOVE:
 	case MADV_WILLNEED:
 	case MADV_DONTNEED:
+	case MADV_DONTNEED_LOCKED:
 	case MADV_COLD:
 	case MADV_PAGEOUT:
 	case MADV_FREE: