RE: [PATCH v4 5/6] drm/gem-shmem: Track folio accessed/dirty status in mmap

Biju Das
2026-03-19T14:53:11Z

Hi Boris,

> -----Original Message-----
> From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of Boris Brezillon
> Sent: 19 March 2026 14:50
> Subject: Re: [PATCH v4 5/6] drm/gem-shmem: Track folio accessed/dirty status in mmap
>
> On Thu, 19 Mar 2026 14:17:31 +0000
> Biju Das <biju.das.jz@bp.renesas.com> wrote:
>
> > Hi All,
> >
> > > -----Original Message-----
> > > From: Biju Das
> > > Sent: 14 March 2026 09:43
> > > Subject: RE: [PATCH v4 5/6] drm/gem-shmem: Track folio
> > > accessed/dirty status in mmap
> > >
> > > Hi Boris Brezillon,
> > >
> > > > -----Original Message-----
> > > > From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On
> > > > Behalf Of Boris Brezillon
> > > > Sent: 13 March 2026 17:46
> > > > Subject: Re: [PATCH v4 5/6] drm/gem-shmem: Track folio
> > > > accessed/dirty status in mmap
> > > >
> > > > On Fri, 13 Mar 2026 13:55:21 +0100 Boris Brezillon
> > > > <boris.brezillon@collabora.com> wrote:
> > > >
> > > > > On Fri, 13 Mar 2026 13:43:28 +0100 Boris Brezillon
> > > > > <boris.brezillon@collabora.com> wrote:
> > > > >
> > > > > > On Fri, 13 Mar 2026 13:18:35 +0100 Boris Brezillon
> > > > > > <boris.brezillon@collabora.com> wrote:
> > > > > >
> > > > > > > On Fri, 13 Mar 2026 12:04:25 +0000 Biju Das
> > > > > > > <biju.das.jz@bp.renesas.com> wrote:
> > > > > > >
> > > > > > > > > -----Original Message-----
> > > > > > > > > From: dri-devel
> > > > > > > > > <dri-devel-bounces@lists.freedesktop.org> On Behalf Of
> > > > > > > > > Boris Brezillon
> > > > > > > > > Sent: 13 March 2026 11:57
> > > > > > > > > Subject: Re: [PATCH v4 5/6] drm/gem-shmem: Track folio
> > > > > > > > > accessed/dirty status in mmap
> > > > > > > > >
> > > > > > > > > On Fri, 13 Mar 2026 11:29:47 +0100 Thomas Zimmermann
> > > > > > > > > <tzimmermann@suse.de> wrote:
> > > > > > > > >
> > > > > > > > > > Hi
> > > > > > > > > >
> > > > > > > > > > Am 13.03.26 um 11:18 schrieb Boris Brezillon:
> > > > > > > > > > [...]
> > > > > > > > > > >>>>> + if (drm_WARN_ON(obj->dev, !shmem->pages || page_offset >= num_pages))
> > > > > > > > > > >>>>> + return VM_FAULT_SIGBUS;
> > > > > > > > > > >>>>> +
> > > > > > > > > > >>>>> + file_update_time(vma->vm_file);
> > > > > > > > > > >>>>> +
> > > > > > > > > > >>>>> +
> > > > > > > > > > >>>>> +folio_mark_dirty(page_folio(shmem->pages[page_o
> > > > > > > > > > >>>>> +ffse
> > > > > > > > > > >>>>> +t]));
> > > > > > > > > > > Do we need a folio_mark_dirty_lock() here?
> > > > > > > > > >
> > > > > > > > > > There is a helper for that with some documentation.
> > > > > > > > > > [1]
> > > > > > > > >
> > > > > > > > > This [1] seems to solve the problem for me. Still unsure
> > > > > > > > > about the folio_mark_dirty_lock vs folio_mark_dirty though.
> > > > > > > > >
> > > > > > > > > [1]https://yhbt.net/lore/dri-devel/20260312155027.168260
> > > > > > > > > 6-1- pedrodemargomes@gmail.com/
> > > > > > > >
> > > > > > > > FYI, I used folio_mark_dirty_lock() still it does not solve the issue with weston hang.
> > > > > > >
> > > > > > > The patch I pointed to has nothing to do with
> > > > > > > folio_mark_dirty_lock(), It's a bug caused by huge page mapping changes.
> > > > > >
> > > > > > Scratch that. I had a bunch of other changes on top, and it
> > > > > > hangs again now that I dropped those.
> > > > >
> > > > > Seems like it's the combination of huge pages and mkwrite that's
> > > > > causing issues, if I disable huge pages, it doesn't hang...
> > > >
> > > > I managed to have it working with the following diff. I still need
> > > > to check why the "map-RO-split+RW- on-demand" approach doesn't
> > > > work (races between huge_fault and pfn_mkwrite?), but I think it's
> > > > okay to map the real thing writable on the first attempt anyway
> > > > (we're not trying to do CoW here, since we're always pointing to
> > > > the same page, it's just the permissions that change). Note that
> > > > there's still the race fixed by
> > > > https://yhbt.net/lore/dri-devel/20260312155027.1682606-1-pedrodema
> > > > rgom es@gmail.com/ in this diff, I just tried to keep the diffstat
> > > > minimal.
> > >
> > > I confirm with this diff, weston is now coming up on RZ/V2L SMARC EVK.
> > >
> > > I am just using drm-misc-next + this diff + arm64 defconfig + Poky
> > > (Yocto Project Reference Distro)
> > > 5.0.11 + Mesa
> > >
> > > Cheers,
> > > Biju
> > >
> > > >
> > > > --->8---
> > > > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > > b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > > index 4500deef4127..4efdce5a60f0 100644
> > > > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > > @@ -561,9 +561,8 @@ static vm_fault_t drm_gem_shmem_try_insert_pfn_pmd(struct vm_fault *vmf,
> unsigne
> > > > bool aligned = (vmf->address & ~PMD_MASK) == (paddr &
> > > > ~PMD_MASK);
> > > >
> > > > if (aligned && pmd_none(*vmf->pmd)) {
> > > > - /* Read-only mapping; split upon write fault */
> > > > pfn &= PMD_MASK >> PAGE_SHIFT;
> > > > - return vmf_insert_pfn_pmd(vmf, pfn, false);
> > > > + return vmf_insert_pfn_pmd(vmf, pfn, vmf->flags &
> > > > + FAULT_FLAG_WRITE);
> > > > }
> > > > #endif
> > > >
> > > > @@ -597,8 +596,12 @@ static vm_fault_t drm_gem_shmem_fault(struct
> > > > vm_fault *vmf)
> > > >
> > > > pfn = page_to_pfn(page);
> > > >
> > > > - if (folio_test_pmd_mappable(folio))
> > > > + if (folio_test_pmd_mappable(folio)) {
> > > > ret = drm_gem_shmem_try_insert_pfn_pmd(vmf, pfn);
> > > > + if (ret == VM_FAULT_NOPAGE && vmf->flags & FAULT_FLAG_WRITE)
> > > > + folio_mark_dirty(folio);
> > > > + }
> > > > +
> > > > if (ret != VM_FAULT_NOPAGE)
> > > > ret = vmf_insert_pfn(vma, vmf->address, pfn);
> > > >
> >
> > Any patch available to fix the weston issue? Still the issue is
> > present with drm-misc-next Please let me know.
>
> Not yet. I want to land [1] first. I'll try to post an official version of the proposed fix tomorrow,
> though I'd really like to have some inputs from the MM maintainers before claiming this is the right
> fix...
>
> [1]https://yhbt.net/lore/dri-devel/20260319015224.46896-1-pedrodemargomes@gmail.com/

Thanks for the update.

Cheers,
Biju