linux-kernel - [PATCH v4 2/2] man/man2/mremap.2: describe previously undocumented shrink behaviour

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Message-ID: <c7ba8ba09b1c0b015134c54824788ef4aa47fd46.1754414738.git.lorenzo.stoakes@oracle.com>
Date: Tue,  5 Aug 2025 18:31:56 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Alejandro Colomar <alx@...nel.org>
Cc: linux-man@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: [PATCH v4 2/2] man/man2/mremap.2: describe previously undocumented shrink behaviour

There is pre-existing logic that appears to be undocumented for an mremap()
shrink operation, where it turns out that the usual 'input range must span
a single mapping' requirement no longer applies.

In fact, it turns out that the input range specified by [old_address,
old_address + old_size) may span any number of mappings.

If shrinking in-place (that is, neither the MREMAP_FIXED nor
MREMAP_DONTUNMAP flags are specified), then the new span may also span any
number of VMAs - [old_address, old_address + new_size).

If shrinking and moving, the range specified by [old_address, old_address +
new_size) must span a single VMA.

There must be at least one VMA contained within the [old_address,
old_address + old_size) range, and old_address must be within the range of
a VMA.

Explicitly document this.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 man/man2/mremap.2 | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
index 6ba51310c..631c835b8 100644
--- a/man/man2/mremap.2
+++ b/man/man2/mremap.2
@@ -48,8 +48,35 @@ The
 .B MREMAP_DONTUNMAP
 flag may be specified.
 .P
-If the operation is not
-simply moving mappings,
+Equally, if the operation performs a shrink,
+that is if
+.I old_size
+is greater than
+.IR new_size ,
+then
+.I old_size
+may also span multiple mappings
+which do not have to be
+adjacent to one another.
+If this shrink is performed
+in-place,
+that is,
+neither
+.BR MREMAP_FIXED ,
+nor
+.B MREMAP_DONTUNMAP
+are specified,
+.I new_size
+may also span multiple VMAs.
+However, if the range is moved,
+then
+.I new_size
+must span only a single mapping.
+.P
+If the operation is neither a
+.B MREMAP_FIXED
+move
+nor a shrink,
 then
 .I old_size
 must span only a single mapping.
-- 
2.50.1