From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Juergen Gross <jgross@suse.com>,
Stefano Stabellini <sstabellini@kernel.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>,
Andrew Morton <akpm@linux-foundation.org>,
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 18:04:37 +0100 [thread overview]
Message-ID: <1b05b9d6-a14c-85cd-0728-d0d40c9ff84b@linux.intel.com> (raw)
In-Reply-To: <20200922163346.GA1701@lst.de>
On 22/09/2020 17:33, Christoph Hellwig wrote:
> 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.
Only reason I can come up with now is if mapping side is on a latency
sensitive path, while un-mapping is lazy/delayed so can be more costly.
Then fast map and extra cost on unmap may make sense.
It more applies to the other i915 patch, which implements a much more
used API, but whether or not we can demonstrate any difference in the
perf profiles I couldn't tell you without trying to collect some.
>> 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?
Not so much given how we don't even use shmem_pin_map outside selftests.
If we start using it I expect it will be for tiny objects anyway. Only
if they end up being pinned for the lifetime of the driver, it may be a
pointless waste of memory compared to the downsides of vmalloc_to_page.
But we can revisit this particular edge case optimization if the need
arises.
I'll look at your other i915 patch tomorrow.
Regards,
Tvrtko
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next prev parent reply other threads:[~2020-09-22 17:04 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-18 16:37 [Intel-gfx] remove alloc_vm_area Christoph Hellwig
2020-09-18 16:37 ` [Intel-gfx] [PATCH 1/6] zsmalloc: switch from alloc_vm_area to get_vm_area Christoph Hellwig
2020-09-21 17:42 ` Minchan Kim
2020-09-21 18:17 ` Christoph Hellwig
2020-09-21 18:42 ` Minchan Kim
2020-09-21 18:43 ` Christoph Hellwig
2020-09-18 16:37 ` [Intel-gfx] [PATCH 2/6] mm: add a vmap_pfn function Christoph Hellwig
2020-09-18 16:37 ` [Intel-gfx] [PATCH 3/6] drm/i915: use vmap in shmem_pin_map Christoph Hellwig
2020-09-21 19:11 ` Matthew Wilcox
2020-09-22 6:22 ` Christoph Hellwig
2020-09-22 8:23 ` Tvrtko Ursulin
2020-09-22 14:31 ` Christoph Hellwig
2020-09-22 16:13 ` Tvrtko Ursulin
2020-09-22 16:33 ` Christoph Hellwig
2020-09-22 17:04 ` Tvrtko Ursulin [this message]
2020-09-23 6:11 ` Christoph Hellwig
2020-09-22 11:21 ` Matthew Wilcox
2020-09-22 14:39 ` Christoph Hellwig
2020-09-22 14:53 ` Matthew Wilcox
2020-09-18 16:37 ` [Intel-gfx] [PATCH 4/6] drm/i915: use vmap in i915_gem_object_map Christoph Hellwig
2020-09-23 9:52 ` Tvrtko Ursulin
2020-09-23 13:41 ` Christoph Hellwig
2020-09-23 13:58 ` Tvrtko Ursulin
2020-09-23 14:44 ` Christoph Hellwig
2020-09-24 12:22 ` Tvrtko Ursulin
2020-09-24 13:23 ` Christoph Hellwig
2020-09-18 16:37 ` [Intel-gfx] [PATCH 5/6] xen/xenbus: use apply_to_page_range directly in xenbus_map_ring_pv Christoph Hellwig
2020-09-18 16:37 ` [Intel-gfx] [PATCH 6/6] x86/xen: open code alloc_vm_area in arch_gnttab_valloc Christoph Hellwig
2020-09-21 20:44 ` boris.ostrovsky
2020-09-22 14:58 ` Christoph Hellwig
2020-09-22 15:24 ` boris.ostrovsky
2020-09-22 15:27 ` Christoph Hellwig
2020-09-22 15:34 ` boris.ostrovsky
2020-09-18 17:03 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for series starting with [1/6] zsmalloc: switch from alloc_vm_area to get_vm_area Patchwork
2020-09-21 17:50 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for series starting with [1/6] zsmalloc: switch from alloc_vm_area to get_vm_area (rev2) Patchwork
2020-09-21 18:47 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for series starting with [1/6] zsmalloc: switch from alloc_vm_area to get_vm_area (rev3) Patchwork
2020-09-22 14:44 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for series starting with [1/6] zsmalloc: switch from alloc_vm_area to get_vm_area (rev4) Patchwork
2020-09-22 15:01 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for series starting with [1/6] zsmalloc: switch from alloc_vm_area to get_vm_area (rev5) Patchwork
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