linux-kernel - [PATCH 1/1] mm/mmap: allow MAP_DROPPABLE

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Message-ID: <20250120012607.4808-1-ioworker0@gmail.com>
Date: Mon, 20 Jan 2025 09:26:07 +0800
From: Lance Yang <ioworker0@...il.com>
To: akpm@...ux-foundation.org
Cc: 21cnbao@...il.com,
	ryan.roberts@....com,
	dev.jain@....com,
	david@...hat.com,
	shy828301@...il.com,
	ziy@...dia.com,
	libang.li@...group.com,
	baolin.wang@...ux.alibaba.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Liam.Howlett@...cle.com,
	lorenzo.stoakes@...cle.com,
	vbabka@...e.cz,
	jannh@...gle.com,
	Jason@...c4.com,
	Lance Yang <ioworker0@...il.com>,
	Mingzhe Yang <mingzhe.yang@...com>
Subject: [PATCH 1/1] mm/mmap: allow MAP_DROPPABLE | MAP_PRIVATE in mmap()

Currently, mmap() fails with `-EINVAL` when both MAP_DROPPABLE and
MAP_PRIVATE are specified. This behavior might be inconsistent, as the
implementation of MAP_DROPPABLE under the hood already includes the
semantics of MAP_PRIVATE. So, IMO, whether MAP_PRIVATE is explicitly
specified or not, it should work as expected.

For example, when mmap() is called with `MAP_DROPPABLE | MAP_ANONYMOUS`,
it creates a private anonymous mapping. Users can verify this behavior
via `/proc/self/smaps`, where the resulting VMA is marked with the `dp`
(MAP_DROPPABLE) flag, and the `Private_*` fields confirm private memory
semantics. The output for a 2MiB mapping with these flags might look like:

```
f433ace00000-f433ad000000 rw-p 00000000 00:00 0
Size:               2048 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                2048 kB
Pss:                2048 kB
Pss_Dirty:          2048 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:      2048 kB
Referenced:         2048 kB
Anonymous:          2048 kB
...
VmFlags: rd wr mr mw me nr wf dd dp
```

This patch changes mmap() to allow the combination of `MAP_DROPPABLE |
MAP_PRIVATE`. For mmap(), at least one of MAP_PRIVATE or MAP_SHARED could
be explicitly specified, regardless of the combination with other `MAP_*`
flags.

Fixes: 9651fcedf7b9 ("mm: add MAP_DROPPABLE for designating always lazily freeable mappings")
Signed-off-by: Mingzhe Yang <mingzhe.yang@...com>
Signed-off-by: Lance Yang <ioworker0@...il.com>
---
 mm/mmap.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index cda01071c7b1..840889b5bfb2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -504,6 +504,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			vm_flags |= VM_SHARED | VM_MAYSHARE;
 			break;
 		case MAP_DROPPABLE:
+		case MAP_DROPPABLE | MAP_PRIVATE:
 			if (VM_DROPPABLE == VM_NONE)
 				return -ENOTSUPP;
 			/*
-- 
2.45.2