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

Biju Das
2026-03-14T09:42:59Z

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_offset]));
> > > > > > > > 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/20260312155027.1682606-1-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-pedrodemargomes@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);
>