From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 88287E7D242 for ; Tue, 26 Sep 2023 07:35:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233516AbjIZHfc (ORCPT ); Tue, 26 Sep 2023 03:35:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56824 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233345AbjIZHf2 (ORCPT ); Tue, 26 Sep 2023 03:35:28 -0400 Received: from madras.collabora.co.uk (madras.collabora.co.uk [IPv6:2a00:1098:0:82:1000:25:2eeb:e5ab]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 11E93F3 for ; Tue, 26 Sep 2023 00:35:21 -0700 (PDT) Received: from localhost (unknown [IPv6:2a01:e0a:2c:6930:5cf4:84a1:2763:fe0d]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by madras.collabora.co.uk (Postfix) with ESMTPSA id 9CEAD6607314; Tue, 26 Sep 2023 08:35:19 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1695713720; bh=ab+8nbg2PMrVtNGAFD8Asd2z04FE9VRGE9dAy3ZJWiA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=EI4lEbMsblThRUsetRRTEVZS6rHCA3vJIXb1uCRxHLX79NVHdT/02VtREn6TgfRDy Fxp7/8tj66psqFO3hANj0kQICCl2dWRebmmjMEhuBRg3LCIkIRK7hf35x3MA+Wltpt zVPYZ4sxgJ5/Bi8EFVCwVXE4VB1lCxqi/2r/7xIb9OG1jkLwsoFR/9l1I5YLa41Jts uoF21hLGsi0ieIcb6BffTw8yFwBi84WlRE1YxG252ayEMNY29+aU0Br2bA3n9SRPJC ht6ras1sy5Q0PMAtFkNlnkD3jmCTA5djdeTl1z74L6CVqOC7cRbZo+WtUH25YWIlr6 6QH8jTeAROGcQ== Date: Tue, 26 Sep 2023 09:35:17 +0200 From: Boris Brezillon To: Dmitry Osipenko Cc: David Airlie , Gerd Hoffmann , Gurchetan Singh , Chia-I Wu , Daniel Vetter , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Christian =?UTF-8?B?S8O2bmln?= , Qiang Yu , Steven Price , Emma Anholt , Melissa Wen , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, kernel@collabora.com, virtualization@lists.linux-foundation.org Subject: Re: [PATCH v17 13/18] drm/shmem-helper: Add memory shrinker Message-ID: <20230926093517.11a172ad@collabora.com> In-Reply-To: <454c464e-4534-7ec3-6d38-49b7df83c7be@collabora.com> References: <20230914232721.408581-1-dmitry.osipenko@collabora.com> <20230914232721.408581-14-dmitry.osipenko@collabora.com> <20230915104633.0d5c3932@collabora.com> <454c464e-4534-7ec3-6d38-49b7df83c7be@collabora.com> Organization: Collabora X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 26 Sep 2023 03:30:35 +0300 Dmitry Osipenko wrote: > On 9/15/23 11:46, Boris Brezillon wrote: > > The naming becomes quite confusing, with drm_gem_shmem_unpin_locked() > > and drm_gem_shmem_unpin_pages_locked(). By the look of it, it seems to > > do exactly the opposite of drm_gem_shmem_swapin_locked(), except for > > the missing ->evicted = true, which we can move here anyway, given > > drm_gem_shmem_purge_locked() explicitly set it to false anyway. The > > other thing that's missing is the > > drm_gem_shmem_update_pages_state_locked(), but it can also be moved > > there I think, if the the ->madv update happens before the > > drm_gem_shmem_unpin_pages_locked() call in > > drm_gem_shmem_purge_locked(). > > > > So, how about renaming this function drm_gem_shmem_swapout_locked()? > > The swapout name would be misleading to me because pages aren't moved to > swap, but allowed to be moved. I'll rename it to > drm_gem_shmem_shrinker_unpin_locked(). If you go this way, I would argue that drm_gem_shmem_swapin_locked() is just as incorrect as drm_gem_shmem_swapout_locked(), in that drm_gem_get_pages() might just return pages that were flagged reclaimable but never reclaimed/swapped-out. I do think that having some symmetry in the naming makes more sense than being 100% accurate. > > >> { > >> struct drm_gem_object *obj = &shmem->base; > >> struct drm_device *dev = obj->dev; > >> > >> dma_resv_assert_held(shmem->base.resv); > >> > >> - drm_WARN_ON(obj->dev, !drm_gem_shmem_is_purgeable(shmem)); > >> + if (shmem->evicted) > >> + return; > >> > >> dma_unmap_sgtable(dev->dev, shmem->sgt, DMA_BIDIRECTIONAL, 0); > > Are we sure we'll always have sgt != NULL? IIRC, if the GEM is only > > mmap-ed in userspace, get_sgt() is not necessarily called by the driver > > (needed to map in GPU space), and we have a potential NULL deref here. > > Maybe that changed at some point in the series, and sgt is > > unconditionally populated when get_pages() is called now. > > The sgt is always set in this function because it's part of shrinker and > shrinker doesn't touch GEMs without sgt. Okay, that's questionable. Why would we not want to reclaim BOs that are only mapped in userspace (sgt == NULL && pages_use_count > 0 && pages_pin_count == 0)? I agree that creating such a BO would be pointless (why create a buffer through DRM if it's not passed to the GPU), but that's still something the API allows... > > >> + __drm_gem_shmem_release_pages(shmem); > > Make sure you drop the implicit pages_use_count ref the sgt had, this > > way you can still tie the necessity to drop the pages to sgt != NULL in > > drm_gem_shmem_free(). > > This will require further refcnt re-initialization when pages are > restored if it's dropped to zero. I don't see how this will improve > anything. Sorry to disagree, but I do think it matters to have a clear ownership model, and if I look at the code (drm_gem_shmem_get_pages_sgt_locked()), the sgt clearly owns a reference to the pages it points to.