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

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Date:   Fri, 11 Jun 2021 00:21:26 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.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 Friday, 11 June 2021 4:04:35 AM AEST Peter Xu wrote:
> External email: Use caution opening links or attachments
> 
> 
> 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?

Yep.

> 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.

Agree that gup calls __split_huge_pmd() via split_huge_pmd_address()
which will always CLEAR. However gup only calls split_huge_pmd_address() if it
finds a thp pmd. In follow_pmd_mask() we have:

	if (likely(!pmd_trans_huge(pmdval)))
		return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);

So I don't think we have a problem here.

> 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
>