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>
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?
> map->flags = vma->vm_flags;
> - *vmap = vma;
> return 0;
> }
>
