Re: [Intel-gfx] [PATCH 3/6] drm/i915: use vmap in shmem_pin_map

From: Christoph Hellwig <hch@lst.de>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Juergen Gross <jgross@suse.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Minchan Kim <minchan@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	intel-gfx@lists.freedesktop.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>,
	Chris Wilson <chris@chris-wilson.co.uk>,
	linux-mm@kvack.org, dri-devel@lists.freedesktop.org,
	xen-devel@lists.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Christoph Hellwig <hch@lst.de>, Nitin Gupta <ngupta@vflare.org>,
	Matthew Auld <matthew.auld@intel.com>
Subject: Re: [Intel-gfx] [PATCH 3/6] drm/i915: use vmap in shmem_pin_map
Date: Tue, 22 Sep 2020 18:33:46 +0200	[thread overview]
Message-ID: <20200922163346.GA1701@lst.de> (raw)
In-Reply-To: <e429c3e6-2143-f51a-4c1c-c1470076ad3e@linux.intel.com>

On Tue, Sep 22, 2020 at 05:13:45PM +0100, Tvrtko Ursulin wrote:
>>   void *shmem_pin_map(struct file *file)
>>   {
>> -	const size_t n_pte = shmem_npte(file);
>> -	pte_t *stack[32], **ptes, **mem;
>
> Chris can comment how much he'd miss the 32 page stack shortcut.

I'd like to see a profile that claim that kmalloc matters in a
path that does a vmap and reads pages through the page cache.
Especially when the kmalloc saves doing another page cache lookup
on the free side.

> Is there something in vmap() preventing us from freeing the pages array 
> here? I can't spot anything that is holding on to the pointer. Or it was 
> just a sketch before you realized we could walk the vm_area?
>
> Also, I may be totally misunderstanding something, but I think you need to 
> assign area->pages manually so shmem_unpin_map can access it below.

We need area->pages to hold the pages for the free side.  That being
said the patch I posted is broken because it never assigned to that.
As said it was a sketch.  This is the patch I just rebooted into on
my Laptop:

http://git.infradead.org/users/hch/misc.git/commitdiff/048522dfa26b6667adfb0371ff530dc263abe829

it needs extra prep patches from the series:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/alloc_vm_area

>>   	mapping_clear_unevictable(file->f_mapping);
>> -	__shmem_unpin_map(file, ptr, shmem_npte(file));
>> +	for (i = 0; i < shmem_npages(file); i++)
>> +		put_page(area->pages[i]);
>> +	kvfree(area->pages);
>> +	vunmap(ptr);
>
> Is the verdict from mm experts that we can't use vfree due __free_pages vs 
> put_page differences?

Switched to vfree now.

> Could we get from ptes to pages, so that we don't have to keep the 
> area->pages array allocated for the duration of the pin?

We could do vmalloc_to_page, but that is fairly expensive (not as bad
as reading from the page cache..).  Are you really worried about the
allocation?
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