linux-kernel - Re: [PATCH v10 07/10] mm: Device exclusive memory access

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Message-ID: <YMJUM4sx1VclR7oz@t490s>
Date:   Thu, 10 Jun 2021 14:04:35 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        rcampbell@...dia.com, linux-doc@...r.kernel.org,
        nouveau@...ts.freedesktop.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        hch@...radead.org, bskeggs@...hat.com, jgg@...dia.com,
        shakeelb@...gle.com, jhubbard@...dia.com, willy@...radead.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v10 07/10] mm: Device exclusive memory access

On Thu, Jun 10, 2021 at 10:18:25AM +1000, Alistair Popple wrote:
> > > The main problem is split_huge_pmd_address() unconditionally calls a mmu
> > > notifier so I would need to plumb in passing an owner everywhere which could
> > > get messy.
> > 
> > Could I ask why?  split_huge_pmd_address() will notify with CLEAR, so I'm a bit
> > confused why we need to pass over the owner.
> 
> Sure, it is the same reason we need to pass it for the exclusive notifier.
> Any invalidation during the make exclusive operation will break the mmu read
> side critical section forcing a retry of the operation. The owner field is what
> is used to filter out invalidations (such as the exclusive invalidation) that
> don't need to be retried.

Do you mean the mmu_interval_read_begin|retry() calls?

Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit
call split_huge_pmd_address(), afaict.  Since both of them use __split_huge_pmd()
internally which will generate that unwanted CLEAR notify.

If that's the case, I think it fails because split_huge_pmd_address() will
trigger that CLEAR notify unconditionally (even if it's not a thp; not sure
whether it should be optimized to not notify at all... definitely another
story), while FOLL_SPLIT_PMD will skip the notify as it calls split_huge_pmd()
instead, who checks the pmd before calling __split_huge_pmd().

Does it also mean that if there's a real THP it won't really work?  As then
FOLL_SPLIT_PMD will start to trigger that CLEAR notify too, I think..

-- 
Peter Xu