Re: [PATCH 1/6] mm: always return EBUSY for invalid ranges in hmm_range_{fault,snapshot}

[Jason Gunthorpe]

On Mon, Jul 22, 2019 at 08:07:33PM +0530, Souptick Joarder wrote:
> On Mon, Jul 22, 2019 at 3:14 PM Christoph Hellwig <hch@xxxxxx> wrote:
> >
> > We should not have two different error codes for the same condition.  In
> > addition this really complicates the code due to the special handling of
> > EAGAIN that drops the mmap_sem due to the FAULT_FLAG_ALLOW_RETRY logic
> > in the core vm.
> >
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > Reviewed-by: Felix Kuehling <Felix.Kuehling@xxxxxxx>
> >  Documentation/vm/hmm.rst |  2 +-
> >  mm/hmm.c                 | 10 ++++------
> >  2 files changed, 5 insertions(+), 7 deletions(-)
> >
> > diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
> > index 7d90964abbb0..710ce1c701bf 100644
> > +++ b/Documentation/vm/hmm.rst
> > @@ -237,7 +237,7 @@ The usage pattern is::
> >        ret = hmm_range_snapshot(&range);
> >        if (ret) {
> >            up_read(&mm->mmap_sem);
> > -          if (ret == -EAGAIN) {
> > +          if (ret == -EBUSY) {
> >              /*
> >               * No need to check hmm_range_wait_until_valid() return value
> >               * on retry we will get proper error with hmm_range_snapshot()
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index e1eedef129cf..16b6731a34db 100644
> > +++ b/mm/hmm.c
> > @@ -946,7 +946,7 @@ EXPORT_SYMBOL(hmm_range_unregister);
> >   * @range: range
> >   * Return: -EINVAL if invalid argument, -ENOMEM out of memory, -EPERM invalid
> >   *          permission (for instance asking for write and range is read only),
> > - *          -EAGAIN if you need to retry, -EFAULT invalid (ie either no valid
> > + *          -EBUSY if you need to retry, -EFAULT invalid (ie either no valid
> >   *          vma or it is illegal to access that range), number of valid pages
> >   *          in range->pfns[] (from range start address).
> >   *
> > @@ -967,7 +967,7 @@ long hmm_range_snapshot(struct hmm_range *range)
> >         do {
> >                 /* If range is no longer valid force retry. */
> >                 if (!range->valid)
> > -                       return -EAGAIN;
> > +                       return -EBUSY;
> >
> >                 vma = find_vma(hmm->mm, start);
> >                 if (vma == NULL || (vma->vm_flags & device_vma))
> > @@ -1062,10 +1062,8 @@ long hmm_range_fault(struct hmm_range *range, bool block)
> >
> >         do {
> >                 /* If range is no longer valid force retry. */
> > -               if (!range->valid) {
> > -                       up_read(&hmm->mm->mmap_sem);
> > -                       return -EAGAIN;
> > -               }
> > +               if (!range->valid)
> > +                       return -EBUSY;
> 
> Is it fine to remove  up_read(&hmm->mm->mmap_sem) ?

It seems very subtle, but under the covers this calls
handle_mm_fault() with FAULT_FLAG_ALLOW_RETRY which will cause the
mmap sem to become unlocked along the -EAGAIN return path.

I think without the commit message I wouldn't have been able to
understand that, so Christoph, could you also add the comment below
please?

Otherwise

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>

Frankly, I find the 'int *locked' scheme of GUP easier to understand..

diff --git a/mm/hmm.c b/mm/hmm.c
index 16b6731a34db79..54b3a4162ae949 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -301,8 +301,10 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
 	flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
 	flags |= write_fault ? FAULT_FLAG_WRITE : 0;
 	ret = handle_mm_fault(vma, addr, flags);
-	if (ret & VM_FAULT_RETRY)
+	if (ret & VM_FAULT_RETRY) {
+		/* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */
 		return -EAGAIN;
+	}
 	if (ret & VM_FAULT_ERROR) {
 		*pfn = range->values[HMM_PFN_ERROR];
 		return -EFAULT;