linux-kernel - Re: [PATCH] mm: add mmap_prepare() compatibility layer for nested file systems

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Message-ID: <30533e96-75f9-4568-add8-05a0be484cfe@suse.cz>
Date: Mon, 9 Jun 2025 12:18:40 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: add mmap_prepare() compatibility layer for nested
 file systems

On 6/9/25 11:24 AM, Lorenzo Stoakes wrote:
> Nested file systems, that is those which invoke call_mmap() within their
> own f_op->mmap() handlers, may encounter underlying file systems which
> provide the f_op->mmap_prepare() hook introduced by commit
> c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback").
> 
> We have a chicken-and-egg scenario here - until all file systems are
> converted to using .mmap_prepare(), we cannot convert these nested
> handlers, as we can't call f_op->mmap from an .mmap_prepare() hook.
> 
> So we have to do it the other way round - invoke the .mmap_prepare() hook
> from an .mmap() one.
> 
> in order to do so, we need to convert VMA state into a struct vm_area_desc
> descriptor, invoking the underlying file system's f_op->mmap_prepare()
> callback passing a pointer to this, and then setting VMA state accordingly
> and safely.
> 
> This patch achieves this via the compat_vma_mmap_prepare() function, which
> we invoke from call_mmap() if f_op->mmap_prepare() is specified in the
> passed in file pointer.
> 
> We place the fundamental logic into mm/vma.c where VMA manipulation
> belongs. We also update the VMA userland tests to accommodate the changes.
> 
> The compat_vma_mmap_prepare() function and its associated machinery is
> temporary, and will be removed once the conversion of file systems is
> complete.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reported-by: Jann Horn <jannh@...gle.com>
> Closes: https://lore.kernel.org/linux-mm/CAG48ez04yOEVx1ekzOChARDDBZzAKwet8PEoPM4Ln3_rk91AzQ@mail.gmail.com/
> Fixes: c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback").

So this is a hotfix for 6.16-rc1 but doesn't need cc: stable.
Also probably nothing wraps yet the filesystems with .mmap_prepare? But
good to have this handled within 6.16.

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>