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 EBFBCC433FE for ; Thu, 17 Mar 2022 17:44:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237005AbiCQRpW (ORCPT ); Thu, 17 Mar 2022 13:45:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45762 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236999AbiCQRpU (ORCPT ); Thu, 17 Mar 2022 13:45:20 -0400 Received: from bhuna.collabora.co.uk (bhuna.collabora.co.uk [IPv6:2a00:1098:0:82:1000:25:2eeb:e3e3]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E9E3FBD7E4 for ; Thu, 17 Mar 2022 10:44:03 -0700 (PDT) Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: dmitry.osipenko) with ESMTPSA id DEB1A1F4552B DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1647539042; bh=mUMMTYKYpCpUpTWTvn3hhHrvuNHT/8z3r7qyrT17pvA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=lJBtpINq0ld1ylJn0JIcyD6BtNED3KoWz7lRZJ7U60pn0clW6BHz5T84ZrDP2ITIO 3Uv19QN0g4XSrEKLLM1vgdGA3cHcR/wBh41nUgXhNbEPAkm3ndYCf0g6mKqG4eBcDR LgOxRmhCfbKISLiqP6E4PEhiFHW5m4iCGOr5XYcZOFTI+OWtPATMhQPiLhLuxtvvoq 8hU7dLwD5MTqtnJaeqDoktCv9+7KXZ3xzfFWZEhDgQ6SKEEe4ZXHQ/TgaRuqh8Ck8T Ogjin72ZPyE+optlFHhoyqLteqiCZIRYxzj0uhk1cRVWuEno4XCGqeBuxFZlysnPD5 yeWmIV9EijnKg== Message-ID: <37364303-acdc-ec95-9e99-2edbc84c5040@collabora.com> Date: Thu, 17 Mar 2022 20:43:57 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH v2 6/8] drm/shmem-helper: Add generic memory shrinker Content-Language: en-US To: Rob Clark Cc: David Airlie , Gerd Hoffmann , Gurchetan Singh , Chia-I Wu , Daniel Vetter , Daniel Almeida , Gert Wollny , Tomeu Vizoso , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Rob Herring , Steven Price , Alyssa Rosenzweig , Linux Kernel Mailing List , "open list:VIRTIO GPU DRIVER" , Gustavo Padovan , dri-devel , Dmitry Osipenko References: <20220314224253.236359-1-dmitry.osipenko@collabora.com> <20220314224253.236359-7-dmitry.osipenko@collabora.com> From: Dmitry Osipenko In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/17/22 19:13, Rob Clark wrote: ... >>>> + /* prevent racing with job submission code paths */ >>>> + if (!dma_resv_trylock(obj->resv)) >>>> + goto shrinker_lock; >>> >>> jfwiw, the trylock here is in the msm code isn't so much for madvise >>> (it is an error to submit jobs that reference DONTNEED objects), but >>> instead for the case of evicting WILLNEED but inactive objects to >>> swap. Ie. in the case that we need to move bo's back in to memory, we >>> don't want to unpin/evict a buffer that is later on the list for the >>> same job.. msm shrinker re-uses the same scan loop for both >>> inactive_dontneed (purge) and inactive_willneed (evict) >> >> I don't see connection between the objects on the shrinker's list and >> the job's BOs. Jobs indeed must not have any objects marked as DONTNEED, >> this case should never happen in practice, but we still need to protect >> from it. > > Hmm, let me try to explain with a simple example.. hopefully this makes sense. > > Say you have a job with two bo's, A and B.. bo A is not backed with > memory (either hasn't been used before or was evicted. Allocating > pages for A triggers shrinker. But B is still on the > inactive_willneed list, however it is already locked (because we don't > want to evict B to obtain backing pages for A). I see now what you're talking about, thanks. My intention of locking the reservations is different since eviction isn't supported by this v2. But we probably will be able to re-use this try_lock for protecting from swapping out job's BOs as well. >>> I suppose using trylock is not technically wrong, and it would be a >>> good idea if the shmem helpers supported eviction as well. But I >>> think in the madvise/purge case if you lose the trylock then there is >>> something else bad going on. >> >> This trylock is intended for protecting job's submission path from >> racing with madvise ioctl invocation followed by immediate purging of >> BOs while job is in a process of submission, i.e. it protects from a >> use-after-free. > > ahh, ok > >> If you'll lose this trylock, then shrinker can't use >> dma_resv_test_signaled() reliably anymore and shrinker may purge BO >> before job had a chance to add fence to the BO's reservation. >> >>> Anyways, from the PoV of minimizing lock contention when under memory >>> pressure, this all looks good to me. >> >> Thank you. I may try to add generic eviction support to the v3. > > eviction is a trickier thing to get right, I wouldn't blame you for > splitting that out into it's own patchset ;-) > > You probably also would want to make it a thing that is opt-in for > drivers using the shmem helpers I had the same thoughts, will see.