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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 d= o > so, they are required to take a reference on that file. > > The mmap() operation which ultimately invokes __mmap_region() is guaran= teed > 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, w= hich > is rather poorly named and unfortunately fails to correctly track wheth= er > 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, keepin= g > 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, deferrin= g > 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 refer= ence > count also must be decremented, so update the logic to call put_map() i= n > this case too. > > Also update __compat_vma_mmap() to manually perform this step for stack= ed > 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) > --- > 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 > #include > #include > #include > 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 =3D mmap_action_prepare(desc); > - if (err) > + if (err) { > + if (desc->vm_file !=3D 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 =3D PHYS_PFN(len_), \ > .vma_flags =3D vma_flags_, \ > .file =3D file_, \ > + .vm_file =3D file_, \ > .page_prot =3D vma_flags_to_page_prot(vma_flags_), \ > } > Best Regards, Yan, Zi