Re: [PATCH v2 05/11] mm/hmm: improve and rename hmm_vma_fault() to hmm_range_fault() v2

Ira Weiny
2019-03-28T21:51:04Z

On Mon, Mar 25, 2019 at 10:40:05AM -0400, Jerome Glisse wrote:
> From: Jérôme Glisse <jglisse@redhat.com>
>
> Rename for consistency between code, comments and documentation. Also
> improves the comments on all the possible returns values. Improve the
> function by returning the number of populated entries in pfns array.
>
> Changes since v1:
> - updated documentation
> - reformated some comments
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> ---
> Documentation/vm/hmm.rst | 8 +---
> include/linux/hmm.h | 13 +++++-
> mm/hmm.c | 91 +++++++++++++++++-----------------------
> 3 files changed, 52 insertions(+), 60 deletions(-)
>
> diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
> index d9b27bdadd1b..61f073215a8d 100644
> --- a/Documentation/vm/hmm.rst
> +++ b/Documentation/vm/hmm.rst
> @@ -190,13 +190,7 @@ When the device driver wants to populate a range of virtual addresses, it can
> use either::
>
> long hmm_range_snapshot(struct hmm_range *range);
> - int hmm_vma_fault(struct vm_area_struct *vma,
> - struct hmm_range *range,
> - unsigned long start,
> - unsigned long end,
> - hmm_pfn_t *pfns,
> - bool write,
> - bool block);
> + long hmm_range_fault(struct hmm_range *range, bool block);
>
> The first one (hmm_range_snapshot()) will only fetch present CPU page table
> entries and will not trigger a page fault on missing or non-present entries.
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 32206b0b1bfd..e9afd23c2eac 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -391,7 +391,18 @@ bool hmm_vma_range_done(struct hmm_range *range);
> *
> * See the function description in mm/hmm.c for further documentation.
> */
> -int hmm_vma_fault(struct hmm_range *range, bool block);
> +long hmm_range_fault(struct hmm_range *range, bool block);
> +
> +/* This is a temporary helper to avoid merge conflict between trees. */
> +static inline int hmm_vma_fault(struct hmm_range *range, bool block)
> +{
> + long ret = hmm_range_fault(range, block);
> + if (ret == -EBUSY)
> + ret = -EAGAIN;
> + else if (ret == -EAGAIN)
> + ret = -EBUSY;
> + return ret < 0 ? ret : 0;
> +}
>
> /* Below are for HMM internal use only! Not to be used by device driver! */
> void hmm_mm_destroy(struct mm_struct *mm);
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 91361aa74b8b..7860e63c3ba7 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -336,13 +336,13 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
> flags |= write_fault ? FAULT_FLAG_WRITE : 0;
> ret = handle_mm_fault(vma, addr, flags);
> if (ret & VM_FAULT_RETRY)
> - return -EBUSY;
> + return -EAGAIN;
> if (ret & VM_FAULT_ERROR) {
> *pfn = range->values[HMM_PFN_ERROR];
> return -EFAULT;
> }
>
> - return -EAGAIN;
> + return -EBUSY;
> }
>
> static int hmm_pfns_bad(unsigned long addr,
> @@ -368,7 +368,7 @@ static int hmm_pfns_bad(unsigned long addr,
> * @fault: should we fault or not ?
> * @write_fault: write fault ?
> * @walk: mm_walk structure
> - * Returns: 0 on success, -EAGAIN after page fault, or page fault error
> + * Returns: 0 on success, -EBUSY after page fault, or page fault error
> *
> * This function will be called whenever pmd_none() or pte_none() returns true,
> * or whenever there is no page directory covering the virtual address range.
> @@ -391,12 +391,12 @@ static int hmm_vma_walk_hole_(unsigned long addr, unsigned long end,
>
> ret = hmm_vma_do_fault(walk, addr, write_fault,
> &pfns[i]);
> - if (ret != -EAGAIN)
> + if (ret != -EBUSY)
> return ret;
> }
> }
>
> - return (fault || write_fault) ? -EAGAIN : 0;
> + return (fault || write_fault) ? -EBUSY : 0;
> }
>
> static inline void hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
> @@ -527,11 +527,11 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> uint64_t orig_pfn = *pfn;
>
> *pfn = range->values[HMM_PFN_NONE];
> - cpu_flags = pte_to_hmm_pfn_flags(range, pte);
> - hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
> - &fault, &write_fault);
> + fault = write_fault = false;
>
> if (pte_none(pte)) {
> + hmm_pte_need_fault(hmm_vma_walk, orig_pfn, 0,
> + &fault, &write_fault);

This really threw me until I applied the patches to a tree. It looks like this
is just optimizing away a pte_none() check. The only functional change which
was mentioned was returning the number of populated pfns. So I spent a bit of
time trying to figure out why hmm_pte_need_fault() needed to move _here_ to do
that... :-(

It would have been nice to have said something about optimizing in the commit
message.

> if (fault || write_fault)
> goto fault;
> return 0;
> @@ -570,7 +570,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> hmm_vma_walk->last = addr;
> migration_entry_wait(vma->vm_mm,
> pmdp, addr);
> - return -EAGAIN;
> + return -EBUSY;
> }
> return 0;
> }
> @@ -578,6 +578,10 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> /* Report error for everything else */
> *pfn = range->values[HMM_PFN_ERROR];
> return -EFAULT;
> + } else {
> + cpu_flags = pte_to_hmm_pfn_flags(range, pte);
> + hmm_pte_need_fault(hmm_vma_walk, orig_pfn, cpu_flags,
> + &fault, &write_fault);

Looks like the same situation as above.

> }
>
> if (fault || write_fault)
> @@ -628,7 +632,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
> if (fault || write_fault) {
> hmm_vma_walk->last = addr;
> pmd_migration_entry_wait(vma->vm_mm, pmdp);
> - return -EAGAIN;
> + return -EBUSY;

While I am at it. Why are we swapping EAGAIN and EBUSY everywhere?

Ira

> }
> return 0;
> } else if (!pmd_present(pmd))
> @@ -856,53 +860,34 @@ bool hmm_vma_range_done(struct hmm_range *range)
> EXPORT_SYMBOL(hmm_vma_range_done);
>
> /*
> - * hmm_vma_fault() - try to fault some address in a virtual address range
> + * hmm_range_fault() - try to fault some address in a virtual address range
> * @range: range being faulted
> * @block: allow blocking on fault (if true it sleeps and do not drop mmap_sem)
> - * Returns: 0 success, error otherwise (-EAGAIN means mmap_sem have been drop)
> + * Returns: number of valid pages in range->pfns[] (from range start
> + * address). This may be zero. If the return value is negative,
> + * then one of the following values may be returned:
> + *
> + * -EINVAL invalid arguments or mm or virtual address are in an
> + * invalid vma (ie either hugetlbfs or device file vma).
> + * -ENOMEM: Out of memory.
> + * -EPERM: Invalid permission (for instance asking for write and
> + * range is read only).
> + * -EAGAIN: If you need to retry and mmap_sem was drop. This can only
> + * happens if block argument is false.
> + * -EBUSY: If the the range is being invalidated and you should wait
> + * for invalidation to finish.
> + * -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).
> *
> * This is similar to a regular CPU page fault except that it will not trigger
> - * any memory migration if the memory being faulted is not accessible by CPUs.
> + * any memory migration if the memory being faulted is not accessible by CPUs
> + * and caller does not ask for migration.
> *
> * On error, for one virtual address in the range, the function will mark the
> * corresponding HMM pfn entry with an error flag.
> - *
> - * Expected use pattern:
> - * retry:
> - * down_read(&mm->mmap_sem);
> - * // Find vma and address device wants to fault, initialize hmm_pfn_t
> - * // array accordingly
> - * ret = hmm_vma_fault(range, write, block);
> - * switch (ret) {
> - * case -EAGAIN:
> - * hmm_vma_range_done(range);
> - * // You might want to rate limit or yield to play nicely, you may
> - * // also commit any valid pfn in the array assuming that you are
> - * // getting true from hmm_vma_range_monitor_end()
> - * goto retry;
> - * case 0:
> - * break;
> - * case -ENOMEM:
> - * case -EINVAL:
> - * case -EPERM:
> - * default:
> - * // Handle error !
> - * up_read(&mm->mmap_sem)
> - * return;
> - * }
> - * // Take device driver lock that serialize device page table update
> - * driver_lock_device_page_table_update();
> - * hmm_vma_range_done(range);
> - * // Commit pfns we got from hmm_vma_fault()
> - * driver_unlock_device_page_table_update();
> - * up_read(&mm->mmap_sem)
> - *
> - * YOU MUST CALL hmm_vma_range_done() AFTER THIS FUNCTION RETURN SUCCESS (0)
> - * BEFORE FREEING THE range struct OR YOU WILL HAVE SERIOUS MEMORY CORRUPTION !
> - *
> - * YOU HAVE BEEN WARNED !
> */
> -int hmm_vma_fault(struct hmm_range *range, bool block)
> +long hmm_range_fault(struct hmm_range *range, bool block)
> {
> struct vm_area_struct *vma = range->vma;
> unsigned long start = range->start;
> @@ -974,7 +959,8 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
> do {
> ret = walk_page_range(start, range->end, &mm_walk);
> start = hmm_vma_walk.last;
> - } while (ret == -EAGAIN);
> + /* Keep trying while the range is valid. */
> + } while (ret == -EBUSY && range->valid);
>
> if (ret) {
> unsigned long i;
> @@ -984,6 +970,7 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
> range->end);
> hmm_vma_range_done(range);
> hmm_put(hmm);
> + return ret;
> } else {
> /*
> * Transfer hmm reference to the range struct it will be drop
> @@ -993,9 +980,9 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
> range->hmm = hmm;
> }
>
> - return ret;
> + return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
> }
> -EXPORT_SYMBOL(hmm_vma_fault);
> +EXPORT_SYMBOL(hmm_range_fault);
> #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
>
>
> --
> 2.17.2
>