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SubjectRe: [PATCH 1/6] mm: always return EBUSY for invalid ranges in hmm_range_{fault,snapshot}
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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@lst.de> 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@lst.de>
> > Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
> > Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
> > 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 /* If range is no longer valid force retry.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 /* If range is no longer valid force retry.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@mellanox.com>

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;
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