Date:   Thu, 28 Mar 2019 20:56:54 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 06/11] mm/hmm: improve driver API to work and wait
 over a range v2

On Thu, Mar 28, 2019 at 09:12:21AM -0700, Ira Weiny wrote:
> On Mon, Mar 25, 2019 at 10:40:06AM -0400, Jerome Glisse wrote:
> > From: Jérôme Glisse <jglisse@...hat.com>
> > 
> > A common use case for HMM mirror is user trying to mirror a range
> > and before they could program the hardware it get invalidated by
> > some core mm event. Instead of having user re-try right away to
> > mirror the range provide a completion mechanism for them to wait
> > for any active invalidation affecting the range.
> > 
> > This also changes how hmm_range_snapshot() and hmm_range_fault()
> > works by not relying on vma so that we can drop the mmap_sem
> > when waiting and lookup the vma again on retry.
> > 
> > Changes since v1:
> >     - squashed: Dan Carpenter: potential deadlock in nonblocking code
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> > Reviewed-by: Ralph Campbell <rcampbell@...dia.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: John Hubbard <jhubbard@...dia.com>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: Dan Carpenter <dan.carpenter@...cle.com>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > ---
> >  include/linux/hmm.h | 208 ++++++++++++++---
> >  mm/hmm.c            | 528 +++++++++++++++++++++-----------------------
> >  2 files changed, 428 insertions(+), 308 deletions(-)
> > 
> > diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> > index e9afd23c2eac..79671036cb5f 100644
> > --- a/include/linux/hmm.h
> > +++ b/include/linux/hmm.h
> > @@ -77,8 +77,34 @@
> >  #include <linux/migrate.h>
> >  #include <linux/memremap.h>
> >  #include <linux/completion.h>
> > +#include <linux/mmu_notifier.h>
> >  
> > -struct hmm;
> > +
> > +/*
> > + * struct hmm - HMM per mm struct
> > + *
> > + * @mm: mm struct this HMM struct is bound to
> > + * @lock: lock protecting ranges list
> > + * @ranges: list of range being snapshotted
> > + * @mirrors: list of mirrors for this mm
> > + * @mmu_notifier: mmu notifier to track updates to CPU page table
> > + * @mirrors_sem: read/write semaphore protecting the mirrors list
> > + * @wq: wait queue for user waiting on a range invalidation
> > + * @notifiers: count of active mmu notifiers
> > + * @dead: is the mm dead ?
> > + */
> > +struct hmm {
> > +	struct mm_struct	*mm;
> > +	struct kref		kref;
> > +	struct mutex		lock;
> > +	struct list_head	ranges;
> > +	struct list_head	mirrors;
> > +	struct mmu_notifier	mmu_notifier;
> > +	struct rw_semaphore	mirrors_sem;
> > +	wait_queue_head_t	wq;
> > +	long			notifiers;
> > +	bool			dead;
> > +};
> >  
> >  /*
> >   * hmm_pfn_flag_e - HMM flag enums
> > @@ -155,6 +181,38 @@ struct hmm_range {
> >  	bool			valid;
> >  };
> >  
> > +/*
> > + * hmm_range_wait_until_valid() - wait for range to be valid
> > + * @range: range affected by invalidation to wait on
> > + * @timeout: time out for wait in ms (ie abort wait after that period of time)
> > + * Returns: true if the range is valid, false otherwise.
> > + */
> > +static inline bool hmm_range_wait_until_valid(struct hmm_range *range,
> > +					      unsigned long timeout)
> > +{
> > +	/* Check if mm is dead ? */
> > +	if (range->hmm == NULL || range->hmm->dead || range->hmm->mm == NULL) {
> > +		range->valid = false;
> > +		return false;
> > +	}
> > +	if (range->valid)
> > +		return true;
> > +	wait_event_timeout(range->hmm->wq, range->valid || range->hmm->dead,
> > +			   msecs_to_jiffies(timeout));
> > +	/* Return current valid status just in case we get lucky */
> > +	return range->valid;
> > +}
> > +
> > +/*
> > + * hmm_range_valid() - test if a range is valid or not
> > + * @range: range
> > + * Returns: true if the range is valid, false otherwise.
> > + */
> > +static inline bool hmm_range_valid(struct hmm_range *range)
> > +{
> > +	return range->valid;
> > +}
> > +
> >  /*
> >   * hmm_pfn_to_page() - return struct page pointed to by a valid HMM pfn
> >   * @range: range use to decode HMM pfn value
> > @@ -357,51 +415,133 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror);
> >  
> >  
> >  /*
> > - * To snapshot the CPU page table, call hmm_vma_get_pfns(), then take a device
> > - * driver lock that serializes device page table updates, then call
> > - * hmm_vma_range_done(), to check if the snapshot is still valid. The same
> > - * device driver page table update lock must also be used in the
> > - * hmm_mirror_ops.sync_cpu_device_pagetables() callback, so that CPU page
> > - * table invalidation serializes on it.
> > + * To snapshot the CPU page table you first have to call hmm_range_register()
> > + * to register the range. If hmm_range_register() return an error then some-
> > + * thing is horribly wrong and you should fail loudly. If it returned true then
> > + * you can wait for the range to be stable with hmm_range_wait_until_valid()
> > + * function, a range is valid when there are no concurrent changes to the CPU
> > + * page table for the range.
> > + *
> > + * Once the range is valid you can call hmm_range_snapshot() if that returns
> > + * without error then you can take your device page table lock (the same lock
> > + * you use in the HMM mirror sync_cpu_device_pagetables() callback). After
> > + * taking that lock you have to check the range validity, if it is still valid
> > + * (ie hmm_range_valid() returns true) then you can program the device page
> > + * table, otherwise you have to start again. Pseudo code:
> > + *
> > + *      mydevice_prefault(mydevice, mm, start, end)
> > + *      {
> > + *          struct hmm_range range;
> > + *          ...
> >   *
> > - * YOU MUST CALL hmm_vma_range_done() ONCE AND ONLY ONCE EACH TIME YOU CALL
> > - * hmm_range_snapshot() WITHOUT ERROR !
> > + *          ret = hmm_range_register(&range, mm, start, end);
> > + *          if (ret)
> > + *              return ret;
> >   *
> > - * IF YOU DO NOT FOLLOW THE ABOVE RULE THE SNAPSHOT CONTENT MIGHT BE INVALID !
> > - */
> > -long hmm_range_snapshot(struct hmm_range *range);
> > -bool hmm_vma_range_done(struct hmm_range *range);
> > -
> > -
> > -/*
> > - * Fault memory on behalf of device driver. Unlike handle_mm_fault(), this will
> > - * not migrate any device memory back to system memory. The HMM pfn array will
> > - * be updated with the fault result and current snapshot of the CPU page table
> > - * for the range.
> > + *          down_read(mm->mmap_sem);
> > + *      again:
> > + *
> > + *          if (!hmm_range_wait_until_valid(&range, TIMEOUT)) {
> > + *              up_read(&mm->mmap_sem);
> > + *              hmm_range_unregister(range);
> > + *              // Handle time out, either sleep or retry or something else
> > + *              ...
> > + *              return -ESOMETHING; || goto again;
> > + *          }
> > + *
> > + *          ret = hmm_range_snapshot(&range); or hmm_range_fault(&range);
> > + *          if (ret == -EAGAIN) {
> > + *              down_read(mm->mmap_sem);
> > + *              goto again;
> > + *          } else if (ret == -EBUSY) {
> > + *              goto again;
> > + *          }
> > + *
> > + *          up_read(&mm->mmap_sem);
> > + *          if (ret) {
> > + *              hmm_range_unregister(range);
> > + *              return ret;
> > + *          }
> > + *
> > + *          // It might not have snap-shoted the whole range but only the first
> > + *          // npages, the return values is the number of valid pages from the
> > + *          // start of the range.
> > + *          npages = ret;
> >   *
> > - * The mmap_sem must be taken in read mode before entering and it might be
> > - * dropped by the function if the block argument is false. In that case, the
> > - * function returns -EAGAIN.
> > + *          ...
> >   *
> > - * Return value does not reflect if the fault was successful for every single
> > - * address or not. Therefore, the caller must to inspect the HMM pfn array to
> > - * determine fault status for each address.
> > + *          mydevice_page_table_lock(mydevice);
> > + *          if (!hmm_range_valid(range)) {
> > + *              mydevice_page_table_unlock(mydevice);
> > + *              goto again;
> > + *          }
> >   *
> > - * Trying to fault inside an invalid vma will result in -EINVAL.
> > + *          mydevice_populate_page_table(mydevice, range, npages);
> > + *          ...
> > + *          mydevice_take_page_table_unlock(mydevice);
> > + *          hmm_range_unregister(range);
> >   *
> > - * See the function description in mm/hmm.c for further documentation.
> > + *          return 0;
> > + *      }
> > + *
> > + * The same scheme apply to hmm_range_fault() (ie replace hmm_range_snapshot()
> > + * with hmm_range_fault() in above pseudo code).
> > + *
> > + * YOU MUST CALL hmm_range_unregister() ONCE AND ONLY ONCE EACH TIME YOU CALL
> > + * hmm_range_register() AND hmm_range_register() RETURNED TRUE ! IF YOU DO NOT
> > + * FOLLOW THIS RULE MEMORY CORRUPTION WILL ENSUE !
> >   */
> > +int hmm_range_register(struct hmm_range *range,
> > +		       struct mm_struct *mm,
> > +		       unsigned long start,
> > +		       unsigned long end);
> > +void hmm_range_unregister(struct hmm_range *range);
> 
> The above comment is great!  But I think you also need to update
> Documentation/vm/hmm.rst:hmm_range_snapshot() to show the use of
> hmm_range_[un]register()
> 
> > +long hmm_range_snapshot(struct hmm_range *range);
> >  long hmm_range_fault(struct hmm_range *range, bool block);
> >  
> > +/*
> > + * HMM_RANGE_DEFAULT_TIMEOUT - default timeout (ms) when waiting for a range
> > + *
> > + * When waiting for mmu notifiers we need some kind of time out otherwise we
> > + * could potentialy wait for ever, 1000ms ie 1s sounds like a long time to
> > + * wait already.
> > + */
> > +#define HMM_RANGE_DEFAULT_TIMEOUT 1000
> > +
> >  /* This is a temporary helper to avoid merge conflict between trees. */
> > +static inline bool hmm_vma_range_done(struct hmm_range *range)
> > +{
> > +	bool ret = hmm_range_valid(range);
> > +
> > +	hmm_range_unregister(range);
> > +	return ret;
> > +}
> > +
> >  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;
> > +	long ret;
> > +
> > +	ret = hmm_range_register(range, range->vma->vm_mm,
> > +				 range->start, range->end);
> > +	if (ret)
> > +		return (int)ret;
> > +
> > +	if (!hmm_range_wait_until_valid(range, HMM_RANGE_DEFAULT_TIMEOUT)) {
> > +		up_read(&range->vma->vm_mm->mmap_sem);
> > +		return -EAGAIN;
> > +	}
> > +
> > +	ret = hmm_range_fault(range, block);
> > +	if (ret <= 0) {
> > +		if (ret == -EBUSY || !ret) {
> > +			up_read(&range->vma->vm_mm->mmap_sem);
> > +			ret = -EBUSY;
> > +		} else if (ret == -EAGAIN)
> > +			ret = -EBUSY;
> > +		hmm_range_unregister(range);
> > +		return ret;
> > +	}
> > +	return 0;
> 
> Is hmm_vma_fault() also temporary to keep the nouveau driver working?  It looks
> like it to me.
> 
> This and hmm_vma_range_done() above are part of the old interface which is in
> the Documentation correct?  As stated above we should probably change that
> documentation with this patch to ensure no new users of these 2 functions
> appear.

Ok will update the documentation, note that i already posted patches to use
this new API see the ODP RDMA link in the cover letter.

Cheers,
Jérôme

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