On 5/21/25 20:20, Lorenzo Stoakes wrote:
> If a user wishes to enable KSM mergeability for an entire process and all
> fork/exec'd processes that come after it, they use the prctl()
> PR_SET_MEMORY_MERGE operation.
>
> This defaults all newly mapped VMAs to have the VM_MERGEABLE VMA flag set
> (in order to indicate they are KSM mergeable), as well as setting this flag
> for all existing VMAs.
>
> However it also entirely and completely breaks VMA merging for the process
> and all forked (and fork/exec'd) processes.
I think merging due to e.g. mprotect() should still work, but for new VMAs,
yeah.
> This is because when a new mapping is proposed, the flags specified will
> never have VM_MERGEABLE set. However all adjacent VMAs will already have
> VM_MERGEABLE set, rendering VMAs unmergeable by default.
>
> To work around this, we try to set the VM_MERGEABLE flag prior to
> attempting a merge. In the case of brk() this can always be done.
>
> However on mmap() things are more complicated - while KSM is not supported
> for file-backed mappings, it is supported for MAP_PRIVATE file-backed
^ insert "shared" to make it obvious?
> mappings.
>
> And these mappings may have deprecated .mmap() callbacks specified which
> could, in theory, adjust flags and thus KSM eligiblity.
Right, however your can_set_ksm_flags_early() isn't testing exactly that?
More on that there.
> This is unlikely to cause an issue on merge, as any adjacent file-backed
> mappings would already have the same post-.mmap() callback attributes, and
> thus would naturally not be merged.
I'm getting a bit lost as two kinds of merging have to be discussed. If the
vma's around have the same afftributes, they would be VMA-merged, no?
> But for the purposes of establishing a VMA as KSM-eligible (as well as
> initially scanning the VMA), this is potentially very problematic.
This part I understand as we have to check if we can add VM_MERGEABLE after
mmap() has adjusted the flags, as it might have an effect on the result of
ksm_compatible()?
> So we check to determine whether this at all possible. If not, we set
> VM_MERGEABLE prior to the merge attempt on mmap(), otherwise we retain the
> previous behaviour.
>
> When .mmap_prepare() is more widely used, we can remove this precaution.
>
> While this doesn't quite cover all cases, it covers a great many (all
> anonymous memory, for instance), meaning we should already see a
> significant improvement in VMA mergeability.
>
> Since, when it comes to file-backed mappings (other than shmem) we are
> really only interested in MAP_PRIVATE mappings which have an available anon
> page by default. Therefore, the VM_SPECIAL restriction makes less sense for
> KSM.
>
> In a future series we therefore intend to remove this limitation, which
> ought to simplify this implementation. However it makes sense to defer
> doing so until a later stage so we can first address this mergeability
> issue.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process") # please no backport!
> Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
<snip>
> +/*
> + * Are we guaranteed no driver can change state such as to preclude KSM merging?
> + * If so, let's set the KSM mergeable flag early so we don't break VMA merging.
> + *
> + * This is applicable when PR_SET_MEMORY_MERGE has been set on the mm_struct via
> + * prctl() causing newly mapped VMAs to have the KSM mergeable VMA flag set.
> + *
> + * If this is not the case, then we set the flag after considering mergeability,
^ "VMA"
> + * which will prevent mergeability as, when PR_SET_MEMORY_MERGE is set, a new
^ "VMA"
> + * VMA will not have the KSM mergeability VMA flag set, but all other VMAs will,
> + * preventing any merge.
^ "VMA"
tedious I know, but more obvious, IMHO
> + */
> +static bool can_set_ksm_flags_early(struct mmap_state *map)
> +{
> + struct file *file = map->file;
> +
> + /* Anonymous mappings have no driver which can change them. */
> + if (!file)
> + return true;
> +
> + /* shmem is safe. */
> + if (shmem_file(file))
> + return true;
> +
> + /*
> + * If .mmap_prepare() is specified, then the driver will have already
> + * manipulated state prior to updating KSM flags.
> + */
> + if (file->f_op->mmap_prepare)
> + return true;
> +
> + return false;
So back to my reply in the commit log, why test for mmap_prepare and
otherwise assume false, and not instead test for f_op->mmap which would
result in false, and otherwise return true? Or am I assuming wrong that
there are f_ops that have neither of those two callbacks?
> +}
> +
> static unsigned long __mmap_region(struct file *file, unsigned long addr,
> unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
> struct list_head *uf)
> @@ -2595,6 +2633,7 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> bool have_mmap_prepare = file && file->f_op->mmap_prepare;
> VMA_ITERATOR(vmi, mm, addr);
> MMAP_STATE(map, mm, &vmi, addr, len, pgoff, vm_flags, file);
> + bool check_ksm_early = can_set_ksm_flags_early(&map);
>
> error = __mmap_prepare(&map, uf);
> if (!error && have_mmap_prepare)
> @@ -2602,6 +2641,9 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> if (error)
> goto abort_munmap;
>
> + if (check_ksm_early)
> + update_ksm_flags(&map);
> +
> /* Attempt to merge with adjacent VMAs... */
> if (map.prev || map.next) {
> VMG_MMAP_STATE(vmg, &map, /* vma = */ NULL);
> @@ -2611,6 +2653,9 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
>
> /* ...but if we can't, allocate a new VMA. */
> if (!vma) {
> + if (!check_ksm_early)
> + update_ksm_flags(&map);
> +
> error = __mmap_new_vma(&map, &vma);
> if (error)
> goto unacct_error;
> @@ -2713,6 +2758,7 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
> * Note: This happens *after* clearing old mappings in some code paths.
> */
> flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> + flags = ksm_vma_flags(mm, NULL, flags);
> if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
> return -ENOMEM;
>
> @@ -2756,7 +2802,6 @@ int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
>
> mm->map_count++;
> validate_mm(mm);
> - ksm_add_vma(vma);
> out:
> perf_event_mmap(vma);
> mm->total_vm += len >> PAGE_SHIFT;
> --
> 2.49.0
