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

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Date:   Tue, 22 Sep 2020 18:33:46 +0200
From:   Christoph Hellwig <hch@....de>
To:     Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-mm@...ck.org, Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
        dri-devel@...ts.freedesktop.org, xen-devel@...ts.xenproject.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        intel-gfx@...ts.freedesktop.org, Nitin Gupta <ngupta@...are.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>
Subject: Re: [Intel-gfx] [PATCH 3/6] drm/i915: use vmap in shmem_pin_map

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?