Re: [PATCH v3 01/40] mm/vma: fix mmap_prepare file handling, remove file_doesnt_need_get

[Zi Yan <ziy@nvidia.com>]

On 17 Sep 2026, at 12:22, Lorenzo Stoakes (ARM) wrote:

> The map->file_doesnt_need_get flag is confusing and the existing
> implementation has holes.
>
> Drivers are permitted to change the owning file of a mapping. If they do
> so, they are required to take a reference on that file.
>
> The mmap() operation which ultimately invokes __mmap_region() is guaranteed
> to drop the refcount for the original file the mapping was made under, but
> this is not true for the replaced file.
>
> This has been addressed so far by tracking map->file_doesnt_need_get, which
> is rather poorly named and unfortunately fails to correctly track whether
> or not an additional put were needed in a number of cases.
>
> Make life easier by removing this flag, and instead drop the reference for
> both mmap_prepare and the deprecated mmap callback in a new function
> put_map().
>
> Track whether this needs to be done by aligning mmap_state with
> vm_area_desc and store the original file in the map->file field, keeping
> the updated file in map->vm_file.
>
> In order to have the same behaviour for both types of hooks, only drop the
> reference __mmap_new_file_vma() itself took in its error path, deferring
> the replaced file's reference to put_map().
>
> To make this work correctly, map->vm_file has to be updated before any
> error handling, so update __mmap_new_file_vma() and call_mmap_prepare() to
> set this field first.
>
> Also when mmap_prepare() changes the file and is then merged, the reference
> count also must be decremented, so update the logic to call put_map() in
> this case too.
>
> Also update __compat_vma_mmap() to manually perform this step for stacked
> file systems using the compatibility layer, and update
> compat_set_vma_from_desc() to replace vma_set_file() with a correct
> refcount/file update.
>
> No in-tree driver is impacted by the incorrect implementation of this
> currently (no driver that does this is mergeable for one), so this does not
> need to be a fix.
>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  mm/internal.h |  1 +
>  mm/util.c     |  5 +++-
>  mm/vma.c      | 83 +++++++++++++++++++++++++++++++++++------------------------
>  mm/vma.h      |  6 +++--
>  4 files changed, 59 insertions(+), 36 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 0dca33db068f..fe576d468af4 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -7,6 +7,7 @@
>  #ifndef __MM_INTERNAL_H
>  #define __MM_INTERNAL_H
>
> +#include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/khugepaged.h>
>  #include <linux/mm.h>
> diff --git a/mm/util.c b/mm/util.c
> index bf0513d1d3d0..016932780925 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1228,8 +1228,11 @@ int __compat_vma_mmap(struct vm_area_desc *desc,
>
>  	/* Perform any preparatory tasks for mmap action. */
>  	err = mmap_action_prepare(desc);
> -	if (err)
> +	if (err) {
> +		if (desc->vm_file != vma->vm_file)
> +			fput(desc->vm_file);
>  		return err;
> +	}
>  	/* Update the VMA from the descriptor. */
>  	compat_set_vma_from_desc(vma, desc);
>  	/* Complete any specified mmap actions. */
> diff --git a/mm/vma.c b/mm/vma.c
> index 55917d097933..fa784f069da4 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -24,7 +24,8 @@ struct mmap_state {
>  		vm_flags_t vm_flags;
>  		vma_flags_t vma_flags;
>  	};
> -	struct file *file;
> +	struct file *file;	/* mmap()-specified file. */

IIUC, file is never assigned other than MMAP_STATE() and should not
change after mmap(). Could it be made const to prevent any change?
I assume mmap_state will not need to handle the nesting issue like
vm_area_desc, so file can be const.

> +	struct file *vm_file;	/* May be updated by mmap_prepare. */
>  	pgprot_t page_prot;
>
>  	/* User-defined fields, perhaps updated by .mmap_prepare(). */
> @@ -43,8 +44,6 @@ struct mmap_state {
>
>  	/* Determine if we can check KSM flags early in mmap() logic. */
>  	bool check_ksm_early :1;
> -	/* If .mmap_prepare changed the file, we don't need to pin. */
> -	bool file_doesnt_need_get :1;
>  };
>
>  #define MMAP_STATE(name, mm_, vmi_, addr_, len_, pgoff_, anon_pgoff_, vma_flags_, file_) \
> @@ -58,6 +57,7 @@ struct mmap_state {
>  		.pglen = PHYS_PFN(len_),				\
>  		.vma_flags = vma_flags_,				\
>  		.file = file_,						\
> +		.vm_file = file_,					\
>  		.page_prot = vma_flags_to_page_prot(vma_flags_),	\
>  	}
>


Best Regards,
Yan, Zi