linux-kernel - Re: [PATCH v2 2/5] mm: add PTE_MARKER

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Message-ID: <1cbc4c4a-a353-4625-ad75-fa32359c99b1@intel.com>
Date: Mon, 21 Oct 2024 14:20:26 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, "Paul E . McKenney"
 <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
 Richard Henderson <richard.henderson@...aro.org>,
 Ivan Kokshaysky <ink@...assic.park.msu.ru>, Matt Turner
 <mattst88@...il.com>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
 Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
 Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
 linux-kselftest@...r.kernel.org, Sidhartha Kumar
 <sidhartha.kumar@...cle.com>, Jeff Xu <jeffxu@...omium.org>,
 Christoph Hellwig <hch@...radead.org>, linux-api@...r.kernel.org,
 John Hubbard <jhubbard@...dia.com>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

On 10/21/24 14:13, Lorenzo Stoakes wrote:
>> Do you think there's enough value int his to warrant digging in? And indeed
>> I imagine bits are in short supply for this and would need a strong
>> argument to get... so yeah I don't think too worthwhile most likely!
>>
>> Thanks for the suggestion though!
> To put it on list - Dave Hansen commented on IRC that it would be safer to
> avoid this for now due to this being an ABI change, and reasonable to
> perhaps add it later if required, so that seems a sensible way forward.

We added SEGV_PKUERR because we really did expect signal handlers to
want to do something new and special, including consuming si_pkey.  Old
signal handlers would probably be pretty confused.

So, yeah, if it's not crystal clear that new signal handler code is
needed, than I'd punt on adding a new SEGV_* code for now.

BTW, SEGV_* codes are sequentially assigned.  It isn't a bitfield and
there are no space constraints.  We've only used a dozen or so of them
and ->si_code is an int.