linux-ext4 - Re: [RFC 04/10] fs/dax: Don't track page mapping/index

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Date: Mon, 15 Apr 2024 17:36:59 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: <linux-mm@...ck.org>, <david@...morbit.com>, <jhubbard@...dia.com>,
	<rcampbell@...dia.com>, <willy@...radead.org>, <jgg@...dia.com>,
	<linux-fsdevel@...r.kernel.org>, <jack@...e.cz>, <djwong@...nel.org>,
	<hch@....de>, <david@...hat.com>, <ruansy.fnst@...itsu.com>,
	<nvdimm@...ts.linux.dev>, <linux-xfs@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>, <jglisse@...hat.com>
Subject: Re: [RFC 04/10] fs/dax: Don't track page mapping/index

Alistair Popple wrote:
> 
> Dan Williams <dan.j.williams@...el.com> writes:
> 
> > Alistair Popple wrote:
> >> I was initially concerned about these cases because I was wondering if
> >> folio subpages could ever get different mappings and the shared case
> >> implied they could. But it seems that's xfs specific and there is a
> >> separate mechanism to deal with looking up ->mapping/index for that. So
> >> I guess we should still be able to safely store this on the folio
> >> head. I will double check and update this change.
> >> 
> >
> > I think there is path to store this information only on the folio head.
> > However, ugh, I think this is potentially another "head" of the
> > pmd_devmap() hydra.
> >
> > pmd_devmap() taught the core-mm to treat dax_pmds indentically to
> > thp_pmds *except* for the __split_huge_pmd() case:
> >
> >    5c7fb56e5e3f mm, dax: dax-pmd vs thp-pmd vs hugetlbfs-pmd
> >
> > Later on pmd migration entries joined pmd_devmap() in skipping splits:
> >
> >    84c3fc4e9c56 mm: thp: check pmd migration entry in common path
> >
> > Unfortunately, pmd_devmap() stopped being considered for skipping
> > splits here:
> >
> >    7f7609175ff2 mm/huge_memory: remove stale locking logic from __split_huge_pmd()
> >
> > Likely __split_huge_pmd_locked() grew support for pmd migration handling
> > and forgot about the pmd_devmap() case.
> >
> > So now Linux has been allowing FSDAX pmd splits since v5.18...
> 
> From what I see we currently (in v6.6) have this in
> __split_huge_pmd_locked():
> 
>         if (!vma_is_anonymous(vma)) {
>                 old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
>                 /*
>                  * We are going to unmap this huge page. So
>                  * just go ahead and zap it
>                  */
>                 if (arch_needs_pgtable_deposit())
>                         zap_deposited_table(mm, pmd);
>                 if (vma_is_special_huge(vma))
>                         return;
> 
> Where vma_is_special_huge(vma) returns true for vma_is_dax(). So AFAICT
> we're still skipping the split right? In all versions we just zap the
> PMD and continue. What am I missing?

Ah, good point I missed that. One more dragon vanquished.