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

Boris Brezillon
2026-03-13T17:46:00Z

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.

--->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);