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Date: Mon, 4 May 2020 17:20:58 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Jason Gunthorpe <jgg@...pe.ca>, <linux-mm@...ck.org>,
Ralph Campbell <rcampbell@...dia.com>
CC: Alex Deucher <alexander.deucher@....com>,
<amd-gfx@...ts.freedesktop.org>, Ben Skeggs <bskeggs@...hat.com>,
Christian König <christian.koenig@....com>,
"David (ChunMing) Zhou" <David1.Zhou@....com>,
<dri-devel@...ts.freedesktop.org>,
Felix Kuehling <Felix.Kuehling@....com>,
Christoph Hellwig <hch@....de>,
<intel-gfx@...ts.freedesktop.org>,
Jérôme Glisse <jglisse@...hat.com>,
<linux-kernel@...r.kernel.org>,
Niranjana Vishwanathapura <niranjana.vishwanathapura@...el.com>,
<nouveau@...ts.freedesktop.org>,
"Yang, Philip" <Philip.Yang@....com>
Subject: Re: [PATCH hmm v2 2/5] mm/hmm: make hmm_range_fault return 0 or -1
On 2020-05-01 11:20, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@...lanox.com>
>
> hmm_vma_walk->last is supposed to be updated after every write to the
> pfns, so that it can be returned by hmm_range_fault(). However, this is
> not done consistently. Fortunately nothing checks the return code of
> hmm_range_fault() for anything other than error.
>
> More importantly last must be set before returning -EBUSY as it is used to
> prevent reading an output pfn as an input flags when the loop restarts.
>
> For clarity and simplicity make hmm_range_fault() return 0 or -ERRNO. Only
> set last when returning -EBUSY.
Yes, this is also a nice simplification.
> ...
> @@ -590,10 +580,13 @@ long hmm_range_fault(struct hmm_range *range)
> return -EBUSY;
> ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
> &hmm_walk_ops, &hmm_vma_walk);
> + /*
> + * When -EBUSY is returned the loop restarts with
> + * hmm_vma_walk.last set to an address that has not been stored
> + * in pfns. All entries < last in the pfn array are set to their
> + * output, and all >= are still at their input values.
> + */
I'm glad you added that comment. This is much easier to figure out with
that in place. After poking around this patch and eventually understanding the
.last handling, I wondered if you might like this slightly tweaked wording
instead:
/*
* Each of the hmm_walk_ops routines returns -EBUSY if and only
* hmm_vma_walk.last has been set to an address that has not yet
* been stored in pfns. All entries < last in the pfn array are
* set to their output, and all >= are still at their input
* values.
*/
Either way,
Reviewed-by: John Hubbard <jhubbard@...dia.com>
thanks,
--
John Hubbard
NVIDIA
> } while (ret == -EBUSY);
> -
> - if (ret)
> - return ret;
> - return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
> + return ret;
> }
> EXPORT_SYMBOL(hmm_range_fault);
>
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