On Fri, Oct 25, 2024 at 11:43:20AM +0200, Vlastimil Babka wrote:
> On 10/23/24 22:38, Lorenzo Stoakes wrote:
> > Rather than trying to merge again when ostensibly allocating a new VMA,
> > instead defer until the VMA is added and attempt to merge the existing
> > range.
> >
> > This way we have no complicated unwinding logic midway through the process
> > of mapping the VMA.
> >
> > In addition this removes limitations on the VMA not being able to be the
> > first in the virtual memory address space which was previously implicitly
> > required.
> >
> > It also performs this merge after the final flag adjustments are performed,
> > something that was not done previously and thus might have prevented
> > possibly valid merges in the past.
> >
> > In theory, for this very same reason, we should unconditionally attempt
> > merge here, however this is likely to have a performance impact so it is
> > better to avoid this given the unlikely outcome of a merge.
>
> Maybe just expand the cases where we set map->retry_merge, i.e. in case the
> final flag adjustments do anything?
>
> > The vmg state will already have been reset by the first attempt at a merge
> > so we only need to reset the iterator, set the vma and flags and try again.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
>
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Thanks (and for 7/8!)
>
> A nit:
>
> > ---
> > mm/vma.c | 75 ++++++++++++++++++++------------------------------------
> > 1 file changed, 26 insertions(+), 49 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 065f5e1f65be..c493ecebf394 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -19,6 +19,7 @@ struct mmap_state {
> > struct file *file;
> >
> > unsigned long charged;
> > + bool retry_merge;
> >
> > struct vm_area_struct *prev;
> > struct vm_area_struct *next;
> > @@ -2280,9 +2281,9 @@ static int __mmap_prepare(struct mmap_state *map, struct vma_merge_struct *vmg,
> > return 0;
> > }
> >
> > +
> > static int __mmap_new_file_vma(struct mmap_state *map,
> > - struct vma_merge_struct *vmg,
> > - struct vm_area_struct **vmap, bool *mergedp)
> > + struct vm_area_struct **vmap)
> > {
> > struct vma_iterator *vmi = map->vmi;
> > struct vm_area_struct *vma = *vmap;
> > @@ -2311,37 +2312,11 @@ static int __mmap_new_file_vma(struct mmap_state *map,
> > !(map->flags & VM_MAYWRITE) &&
> > (vma->vm_flags & VM_MAYWRITE));
> >
> > - vma_iter_config(vmi, map->addr, map->end);
> > - /*
> > - * If flags changed after mmap_file(), we should try merge
> > - * vma again as we may succeed this time.
> > - */
> > - if (unlikely(map->flags != vma->vm_flags && map->prev)) {
> > - struct vm_area_struct *merge;
> > -
> > - vmg->flags = vma->vm_flags;
> > - /* If this fails, state is reset ready for a reattempt. */
> > - merge = vma_merge_new_range(vmg);
> > -
> > - if (merge) {
> > - /*
> > - * ->mmap() can change vma->vm_file and fput
> > - * the original file. So fput the vma->vm_file
> > - * here or we would add an extra fput for file
> > - * and cause general protection fault
> > - * ultimately.
> > - */
> > - fput(vma->vm_file);
> > - vm_area_free(vma);
> > - vma = merge;
> > - *mergedp = true;
> > - } else {
> > - vma_iter_config(vmi, map->addr, map->end);
> > - }
> > - }
> > + /* If the flags change (and are mergeable), let's retry later. */
> > + map->retry_merge = vma->vm_flags != map->flags && !(vma->vm_flags & VM_SPECIAL);
> >
> > + vma_iter_config(vmi, map->addr, map->end);
>
> Do we need this still? __mmap_new_vma() did that and nothing changed since
> in the non-error case, AFAICS?
You're right, this change really highlights that, will remove thanks!
>
> > map->flags = vma->vm_flags;
> > - *vmap = vma;
> > return 0;
> > }
> >
