[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <764d413a-43a3-4be2-99c4-616cd8cd3998@lucifer.local>
Date: Mon, 8 Sep 2025 15:47:34 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
Guo Ren <guoren@...nel.org>,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>,
"David S . Miller" <davem@...emloft.net>,
Andreas Larsson <andreas@...sler.com>, Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Dan Williams <dan.j.williams@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
Dave Jiang <dave.jiang@...el.com>, Nicolas Pitre <nico@...xnic.net>,
Muchun Song <muchun.song@...ux.dev>,
Oscar Salvador <osalvador@...e.de>,
David Hildenbrand <david@...hat.com>,
Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
Dave Young <dyoung@...hat.com>, Tony Luck <tony.luck@...el.com>,
Reinette Chatre <reinette.chatre@...el.com>,
Dave Martin <Dave.Martin@....com>, James Morse <james.morse@....com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
Hugh Dickins <hughd@...gle.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
Uladzislau Rezki <urezki@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>, Jann Horn <jannh@...gle.com>,
Pedro Falcato <pfalcato@...e.de>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-csky@...r.kernel.org, linux-mips@...r.kernel.org,
linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
nvdimm@...ts.linux.dev, linux-cxl@...r.kernel.org, linux-mm@...ck.org,
ntfs3@...ts.linux.dev, kexec@...ts.infradead.org,
kasan-dev@...glegroups.com
Subject: Re: [PATCH 03/16] mm: add vma_desc_size(), vma_desc_pages() helpers
On Mon, Sep 08, 2025 at 11:20:11AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 03:09:43PM +0100, Lorenzo Stoakes wrote:
> > > Perhaps
> > >
> > > !vma_desc_cowable()
> > >
> > > Is what many drivers are really trying to assert.
> >
> > Well no, because:
> >
> > static inline bool is_cow_mapping(vm_flags_t flags)
> > {
> > return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
> > }
> >
> > Read-only means !CoW.
>
> What drivers want when they check SHARED is to prevent COW. It is COW
> that causes problems for whatever the driver is doing, so calling the
> helper cowable and making the test actually right for is a good thing.
>
> COW of this VMA, and no possibilty to remap/mprotect/fork/etc it into
> something that is COW in future.
But you can't do that if !VM_MAYWRITE.
I mean probably the driver's just wrong and should use is_cow_mapping() tbh.
>
> Drivers have commonly various things with VM_SHARED to establish !COW,
> but if that isn't actually right then lets fix it to be clear and
> correct.
I think we need to be cautious of scope here :) I don't want to accidentally
break things this way.
OK I think a sensible way forward - How about I add desc_is_cowable() or
vma_desc_cowable() and only set this if I'm confident it's correct?
That way I can achieve both aims at once.
>
> Jason
Cheers, Lorenzo
Powered by blists - more mailing lists