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Date:   Sun, 25 Jun 2023 02:51:43 -0700
From:   Sumitra Sharma <sumitraartsy@gmail.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     Patrik Jakobsson <patrik.r.jakobsson@gmail.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Ira Weiny <ira.weiny@intel.com>,
        Deepak R Varma <drv@mailo.com>, zhao1.liu@intel.com,
        Zhao Liu <zhao1.liu@linux.intel.com>,
        Sumitra Sharma <sumitraartsy@gmail.com>
Subject: Re: [PATCH] drm/gma500: Replace kmap{,_atomic}() with page_address()
Message-ID: <20230625095143.GB436009@sumitra.com>
References: <20230620180148.GA419134@sumitra.com>
 <2565983.Lt9SDvczpP@suse>
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On Wed, Jun 21, 2023 at 12:31:40AM +0200, Fabio M. De Francesco wrote:
> On marted? 20 giugno 2023 20:01:48 CEST Sumitra Sharma wrote:
> > Remove unnecessary calls to kmap{,_atomic}() when acquiring
> > pages using GFP_DMA32.
> > 
> > The GFP_DMA32 uses the DMA32 zone to satisfy the allocation
> > requests. Therefore, pages allocated with GFP_DMA32 cannot
> > come from Highmem.
> > 
> > Avoid using calls to kmap_local_page() / kunmap_local() and
> > kmap() / kunmap() in the psb_mmu_alloc_pd function. Instead,
> > utilize page_address().
> > 
> > Remove the usage of kmap_atomic() / kunmap_atomic() in the
> > psb_mmu_alloc_pt function. Use page_address() instead.
> > 
> > Substitute kmap_atomic(pt->p) / kunmap_atomic(pt->v) calls
> > in the psb_mmu_pt_alloc_map_lock() and psb_mmu_pt_unmap_unlock()
> > functions with page_address(pt->p). This is possible as
> > pt = psb_mmu_alloc_pt(pd) allocates a page using
> > pt->p = alloc_page(GFP_DMA32).
> 
> Sumitra,
> 
> I'm sorry because this patch cannot acked with this commit message.
> 
> This commit message is missing two _really_ important information. Therefore, 
> it is not acked. Please check again what I write below and either add the 
> missing information or change the code accordingly...
> 
> You should assure everybody that the code between the old kmap_atomic() / 
> kunmap_atomic() doesn't depend either on implicit pagefault_disable() or 
> preempt_disable() calls or both. 
> 
> Please read again the section of the Highmem's documentation regarding 
> kmap_atomic() at https://docs.kernel.org/mm/highmem.html
> 
> In particular take care to read and understand what "[] the code between calls 
> to kmap_atomic() and kunmap_atomic() may implicitly depend on the side effects 
> of atomic mappings, i.e. disabling page faults or preemption, or both. In that 
> case, explicit calls to pagefault_disable() or preempt_disable() or both must 
> be made in conjunction with the use of kmap_local_page().".
> 
> Please study carefully also the following patch from Zhao, suggested by Ira 
> and reviewed by Ira and I: "[PATCH v2 3/9] drm/i915: Use kmap_local_page() in 
> gem/i915_gem_shmem.c". It's not yet reached upstream so you need to find it in 
> lore.kernel.org at https://lore.kernel.org/lkml/20230329073220.3982460-4-zhao1.liu@linux.intel.com/
> 
> Please note that, in turn, that patch also contains a link to a patch from Ira 
> who too had to disable faults (https://lore.kernel.org/all/
> 20220813220034.806698-1-ira.weiny@intel.com)
> 
> I haven't yet looked at your code. If any sections do depend on those implicit 
> disables, you should act accordingly and add one or both of the above-
> mentioned calls, even in cases where you get rid of local mappings.
> 
> Instead if the sections don't depend on the mentioned side effects, you should 
> write something like what I wrote in "[PATCH] NFS: Convert kmap_atomic() to 
> kmap_local_folio()" that you can find at https://lore.kernel.org/lkml/
> 20230503172411.3356-1-fmdefrancesco@gmail.com/ or, by by using "git show 
> 4b71e2416ec4".
> 
> Thanks for working on this,
> 
> Fabio 
> 


Hi Fabio,

Thank you for notifying the mistakes I made in this patch. I am directly
working on the changes.

Regards
Sumitra


> > 
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Sumitra Sharma <sumitraartsy@gmail.com>
> > ---
> >  drivers/gpu/drm/gma500/mmu.c | 17 +++++------------
> >  1 file changed, 5 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/gma500/mmu.c b/drivers/gpu/drm/gma500/mmu.c
> > index a70b01ccdf70..59aa5661e56a 100644
> > --- a/drivers/gpu/drm/gma500/mmu.c
> > +++ b/drivers/gpu/drm/gma500/mmu.c
> > @@ -184,20 +184,15 @@ struct psb_mmu_pd *psb_mmu_alloc_pd(struct
> > psb_mmu_driver *driver, pd->invalid_pte = 0;
> >  	}
> > 
> > -	v = kmap_local_page(pd->dummy_pt);
> > +	v = page_address(pd->dummy_pt);
> >  	for (i = 0; i < (PAGE_SIZE / sizeof(uint32_t)); ++i)
> >  		v[i] = pd->invalid_pte;
> > 
> > -	kunmap_local(v);
> > -
> > -	v = kmap_local_page(pd->p);
> > +	v = page_address(pd->p);
> >  	for (i = 0; i < (PAGE_SIZE / sizeof(uint32_t)); ++i)
> >  		v[i] = pd->invalid_pde;
> > 
> > -	kunmap_local(v);
> > -
> > -	clear_page(kmap(pd->dummy_page));
> > -	kunmap(pd->dummy_page);
> > +	clear_page(page_address(pd->dummy_page));
> > 
> >  	pd->tables = vmalloc_user(sizeof(struct psb_mmu_pt *) * 1024);
> >  	if (!pd->tables)
> > @@ -279,7 +274,7 @@ static struct psb_mmu_pt *psb_mmu_alloc_pt(struct
> > psb_mmu_pd *pd)
> > 
> >  	spin_lock(lock);
> > 
> > -	v = kmap_atomic(pt->p);
> > +	v = page_address(pt->p);
> >  	clf = (uint8_t *) v;
> >  	ptes = (uint32_t *) v;
> >  	for (i = 0; i < (PAGE_SIZE / sizeof(uint32_t)); ++i)
> > @@ -293,7 +288,6 @@ static struct psb_mmu_pt *psb_mmu_alloc_pt(struct
> > psb_mmu_pd *pd) }
> >  		mb();
> >  	}
> > -	kunmap_atomic(v);
> >  	spin_unlock(lock);
> > 
> >  	pt->count = 0;
> > @@ -339,7 +333,7 @@ static struct psb_mmu_pt 
> *psb_mmu_pt_alloc_map_lock(struct
> > psb_mmu_pd *pd, atomic_set(&pd->driver->needs_tlbflush, 1);
> >  		}
> >  	}
> > -	pt->v = kmap_atomic(pt->p);
> > +	pt->v = page_address(pt->p);
> >  	return pt;
> >  }
> > 
> > @@ -365,7 +359,6 @@ static void psb_mmu_pt_unmap_unlock(struct psb_mmu_pt 
> *pt)
> > struct psb_mmu_pd *pd = pt->pd;
> >  	uint32_t *v;
> > 
> > -	kunmap_atomic(pt->v);
> >  	if (pt->count == 0) {
> >  		v = kmap_atomic(pd->p);
> >  		v[pt->index] = pd->invalid_pde;
> > --
> > 2.25.1
> 
> 
> 
>