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Message-ID: <lus7wfr2fcycylium7ljykdbywinsfmaow45xhiduiitajzclj@s5pzxkvyd6fd>
Date: Mon, 9 Jun 2025 12:35:40 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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>,
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 Mon, Jun 09, 2025 at 10:24:13AM +0100, 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.
>
Thanks, this is annoying but looks mostly cromulent!
> 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").
> ---
> include/linux/fs.h | 6 +++--
> mm/mmap.c | 39 +++++++++++++++++++++++++++
> mm/vma.c | 46 +++++++++++++++++++++++++++++++-
> mm/vma.h | 4 +++
> tools/testing/vma/vma_internal.h | 16 +++++++++++
> 5 files changed, 108 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 05abdabe9db7..8fe41a2b7527 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2274,10 +2274,12 @@ static inline bool file_has_valid_mmap_hooks(struct file *file)
> return true;
> }
>
> +int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma);
> +
> static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
> {
> - if (WARN_ON_ONCE(file->f_op->mmap_prepare))
> - return -EINVAL;
> + if (file->f_op->mmap_prepare)
> + return compat_vma_mmap_prepare(file, vma);
>
> return file->f_op->mmap(file, vma);
> }
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 09c563c95112..0755cb5d89d1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1891,3 +1891,42 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
> vm_unacct_memory(charge);
> goto loop_out;
> }
> +
> +/**
> + * compat_vma_mmap_prepare() - Apply the file's .mmap_prepare() hook to an
> + * existing VMA
> + * @file: The file which possesss an f_op->mmap_prepare() hook
> + * @vma; The VMA to apply the .mmap_prepare() hook to.
> + *
> + * Ordinarily, .mmap_prepare() is invoked directly upon mmap(). However, certain
> + * 'wrapper' file systems invoke a nested mmap hook of an underlying file.
> + *
> + * Until all filesystems are converted to use .mmap_prepare(), we must be
> + * conservative and continue to invoke these 'wrapper' filesystems using the
> + * deprecated .mmap() hook.
> + *
> + * However we have a problem if the underlying file system possesses an
> + * .mmap_prepare() hook, as we are in a different context when we invoke the
> + * .mmap() hook, already having a VMA to deal with.
> + *
> + * compat_vma_mmap_prepare() is a compatibility function that takes VMA state,
> + * establishes a struct vm_area_desc descriptor, passes to the underlying
> + * .mmap_prepare() hook and applies any changes performed by it.
> + *
> + * Once the conversion of filesystems is complete this function will no longer
> + * be required and will be removed.
> + *
> + * Returns: 0 on success or error.
> + */
> +int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
> +{
> + struct vm_area_desc desc;
> + int err;
> +
> + err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
> + if (err)
> + return err;
> + set_vma_from_desc(vma, &desc);
> +
> + return 0;
> +}
> diff --git a/mm/vma.c b/mm/vma.c
> index 01b1d26d87b4..d771750f8f76 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -3153,7 +3153,6 @@ int __vm_munmap(unsigned long start, size_t len, bool unlock)
> return ret;
> }
>
> -
> /* Insert vm structure into process list sorted by address
> * and into the inode's i_mmap tree. If vm_file is non-NULL
> * then i_mmap_rwsem is taken here.
> @@ -3195,3 +3194,48 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
>
> return 0;
> }
> +
> +/*
> + * Temporary helper functions for file systems which wrap an invocation of
> + * f_op->mmap() but which might have an underlying file system which implements
> + * f_op->mmap_prepare().
> + */
> +
> +struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
> + struct vm_area_desc *desc)
> +{
> + desc->mm = vma->vm_mm;
> + desc->start = vma->vm_start;
> + desc->end = vma->vm_end;
> +
> + desc->pgoff = vma->vm_pgoff;
> + desc->file = vma->vm_file;
> + desc->vm_flags = vma->vm_flags;
> + desc->page_prot = vma->vm_page_prot;
> +
> + desc->vm_ops = NULL;
> + desc->private_data = NULL;
> +
> + return desc;
> +}
> +
> +void set_vma_from_desc(struct vm_area_struct *vma, struct vm_area_desc *desc)
> +{
> + /*
> + * Since we're invoking .mmap_prepare() despite having a partially
> + * established VMA, we must take care to handle setting fields
> + * correctly.
> + */
> +
> + /* Mutable fields. Populated with initial state. */
> + vma->vm_pgoff = desc->pgoff;
> + if (vma->vm_file != desc->file)
> + vma_set_file(vma, desc->file);
> + if (vma->vm_flags != desc->vm_flags)
> + vm_flags_set(vma, desc->vm_flags);
I think we don't need vm_flags_set in this case, since the VMA isn't exposed yet.
__vm_flags_mod should work just fine. Of course this isn't a big deal, but I would
like it if we reduced vm_flags_set to core mm and conceptually attached things.
In any case, with or without that addressed:
Reviewed-by: Pedro Falcato <pfalcato@...e.de>
--
Pedro
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