On Fri, Feb 26, 2021 at 06:18:27PM +1100, Alistair Popple wrote:
> The behaviour of try_to_unmap_one() is difficult to follow because it
> performs different operations based on a fairly large set of flags used
> in different combinations.
>
> TTU_MUNLOCK is one such flag. However it is exclusively used by
> try_to_munlock() which specifies no other flags. Therefore rather than
> overload try_to_unmap_one() with unrelated behaviour split this out into
> it's own function and remove the flag.
>
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
>
>
> Given the comments on not needing to hold mmap_lock it was not 100% clear
> to me if it is safe to check vma->vma_flags & VM_LOCKED and if re-checking
> under the ptl was significant. I left the extra check in case it was, but
> it seems one of the checks is redunant as either the first check is racey
> or the second check is unneccsary.
The rmap doesn't hold the mmap_lock so I think both of these cases are
racey.
eg
apply_vma_lock_flags()
vma = find_vma(current->mm, start);
if (!vma || vma->vm_start > start)
return -ENOMEM;
prev = vma->vm_prev;
if (start > vma->vm_start)
prev = vma;
for (nstart = start ; ; ) {
vm_flags_t newflags = vma->vm_flags & VM_LOCKED_CLEAR_MASK;
newflags |= flags;
[...]
mlock_fixup()
/*
* vm_flags is protected by the mmap_lock held in write mode.
* It's okay if try_to_unmap_one unmaps a page just after we
* set VM_LOCKED, populate_vma_page_range will bring it back.
*/
if (lock)
vma->vm_flags = newflags;
else
vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
Which is only done under the mmap_sem
> +static bool try_to_munlock_one(struct page *page, struct vm_area_struct *vma,
> + unsigned long address, void *arg)
> +{
> + struct page_vma_mapped_walk pvmw = {
> + .page = page,
> + .vma = vma,
> + .address = address,
> + };
> + bool ret = true;
> +
> + /* munlock has nothing to gain from examining un-locked vmas */
> + if (!(vma->vm_flags & VM_LOCKED))
> + return true;
The mmap_sem can't be obtained in the rmap walkers due to lock
ordering, the various rmap locks are nested under the mmap_sem
So, when reading data that is not locked it should be written as:
READ_ONCE(vma->vm_flags) & VM_LOCKED
> + while (page_vma_mapped_walk(&pvmw)) {
> + /*
> + * If the page is mlock()d, we cannot swap it out.
> + * If it's recently referenced (perhaps page_referenced
> + * skipped over this mm) then we should reactivate it.
> + */
> + if (vma->vm_flags & VM_LOCKED) {
And since we write the data without holding the PTLs this looks
pointless, unless there is some other VM_LOCKED manipulation
Jason
