linux-kernel - Re: [PATCH v6 23/26] mm: Remove pXX

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <67a2652083600_2d2c2942f@dwillia2-xfh.jf.intel.com.notmuch>
Date: Tue, 4 Feb 2025 11:06:08 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<alison.schofield@...el.com>, <lina@...hilina.net>, <zhang.lyra@...il.com>,
	<gerald.schaefer@...ux.ibm.com>, <vishal.l.verma@...el.com>,
	<dave.jiang@...el.com>, <logang@...tatee.com>, <bhelgaas@...gle.com>,
	<jack@...e.cz>, <jgg@...pe.ca>, <catalin.marinas@....com>, <will@...nel.org>,
	<mpe@...erman.id.au>, <npiggin@...il.com>, <dave.hansen@...ux.intel.com>,
	<ira.weiny@...el.com>, <willy@...radead.org>, <djwong@...nel.org>,
	<tytso@....edu>, <linmiaohe@...wei.com>, <david@...hat.com>,
	<peterx@...hat.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linuxppc-dev@...ts.ozlabs.org>, <nvdimm@...ts.linux.dev>,
	<linux-cxl@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
	<jhubbard@...dia.com>, <hch@....de>, <david@...morbit.com>,
	<chenhuacai@...nel.org>, <kernel@...0n.name>, <loongarch@...ts.linux.dev>
Subject: Re: [PATCH v6 23/26] mm: Remove pXX_devmap callers

Alistair Popple wrote:
> On Tue, Jan 14, 2025 at 10:50:49AM -0800, Dan Williams wrote:
> > Alistair Popple wrote:
> > > The devmap PTE special bit was used to detect mappings of FS DAX
> > > pages. This tracking was required to ensure the generic mm did not
> > > manipulate the page reference counts as FS DAX implemented it's own
> > > reference counting scheme.
> > > 
> > > Now that FS DAX pages have their references counted the same way as
> > > normal pages this tracking is no longer needed and can be
> > > removed.
> > > 
> > > Almost all existing uses of pmd_devmap() are paired with a check of
> > > pmd_trans_huge(). As pmd_trans_huge() now returns true for FS DAX pages
> > > dropping the check in these cases doesn't change anything.
> > > 
> > > However care needs to be taken because pmd_trans_huge() also checks that
> > > a page is not an FS DAX page. This is dealt with either by checking
> > > !vma_is_dax() or relying on the fact that the page pointer was obtained
> > > from a page list. This is possible because zone device pages cannot
> > > appear in any page list due to sharing page->lru with page->pgmap.
> > 
> > While the patch looks straightforward I think part of taking "care" in
> > this case is to split it such that any of those careful conversions have
> > their own bisect point in the history.
> > 
> > Perhaps this can move to follow-on series to not blow up the patch count
> > of the base series? ...but first want to get your reaction to splitting
> > for bisect purposes.
> 
> TBH I don't feel too strongly about it - I suppose it would make it easier to
> bisect to the specific case we weren't careful enough about. However I think if
> a bug is bisected to this particular patch it would be relatively easy based on
> the context of the bug to narrow it down to a particular file or two.
> 
> I do however feel strongly about whether or not that should be done in a
> follow-on series :-)
> 
> Rebasing such a large series has already become painful and error prone enough
> so if we want to split this change up it will definitely need to be a separate
> series done once the rest of this has been merged. So I could be pursaded to
> roll this and the pfn_t removal (as that depends on devmap going away) together.
> 
> Let me know what you think.

I tend to think that there's never any regrets for splitting a patch
along lines of risk. I am fine with keeping that in this series if that
makes things easier.