linux-ext4 - Re: [PATCH 00/13] fs/dax: Fix FS DAX page reference counts

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Message-ID: <ZoIvhDvzMCw28VBI@dread.disaster.area>
Date: Mon, 1 Jul 2024 14:24:36 +1000
From: Dave Chinner <david@...morbit.com>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, vishal.l.verma@...el.com,
	dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
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Subject: Re: [PATCH 00/13] fs/dax: Fix FS DAX page reference counts

On Thu, Jun 27, 2024 at 10:54:15AM +1000, Alistair Popple wrote:
> FS DAX pages have always maintained their own page reference counts
> without following the normal rules for page reference counting. In
> particular pages are considered free when the refcount hits one rather
> than zero and refcounts are not added when mapping the page.
> 
> Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
> mechanism for allowing GUP to hold references on the page (see
> get_dev_pagemap). However there doesn't seem to be any reason why FS
> DAX pages need their own reference counting scheme.
> 
> By treating the refcounts on these pages the same way as normal pages
> we can remove a lot of special checks. In particular pXd_trans_huge()
> becomes the same as pXd_leaf(), although I haven't made that change
> here. It also frees up a valuable SW define PTE bit on architectures
> that have devmap PTE bits defined.
> 
> It also almost certainly allows further clean-up of the devmap managed
> functions, but I have left that as a future improvment.
> 
> This is an update to the original RFC rebased onto v6.10-rc5. Unlike
> the original RFC it passes the same number of ndctl test suite
> (https://github.com/pmem/ndctl) tests as my current development
> environment does without these patches.

I strongly suggest running fstests on pmem devices with '-o
dax=always' mount options to get much more comprehensive fsdax test
coverage. That exercises a lot of the weird mmap corner cases that
cause problems so it would be good to actually test that nothing new
got broken in FSDAX by this patchset.

-Dave.
-- 
Dave Chinner
david@...morbit.com