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Tue, 01 Nov 2022 15:06:02 -0700 (PDT) MIME-Version: 1.0 References: <20221031183122.470962-1-shy828301@gmail.com> In-Reply-To: From: "Zach O'Keefe" Date: Tue, 1 Nov 2022 15:05:26 -0700 Message-ID: Subject: Re: [PATCH] mm: don't warn if the node is offlined To: Yang Shi Cc: Michal Hocko , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 1, 2022 at 1:09 PM Yang Shi wrote: > > On Tue, Nov 1, 2022 at 12:14 PM Zach O'Keefe wrote: > > > > On Tue, Nov 1, 2022 at 10:13 AM Yang Shi wrote: > > > > > > On Tue, Nov 1, 2022 at 12:54 AM Michal Hocko wrote: > > > > > > > > On Mon 31-10-22 17:05:06, Zach O'Keefe wrote: > > > > > On Mon, Oct 31, 2022 at 3:08 PM Michal Hocko wrote: > > > > > > > > > > > > On Mon 31-10-22 11:31:22, Yang Shi wrote: > > > > > > > Syzbot reported the below splat: > > > > > > > > > > > > > > WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 __alloc_pages_node include/linux/gfp.h:221 [inline] > > > > > > > WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 hpage_collapse_alloc_page mm/khugepaged.c:807 [inline] > > > > > > > WARNING: CPU: 1 PID: 3646 at include/linux/gfp.h:221 alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963 > > > > > > > Modules linked in: > > > > > > > CPU: 1 PID: 3646 Comm: syz-executor210 Not tainted 6.1.0-rc1-syzkaller-00454-ga70385240892 #0 > > > > > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022 > > > > > > > RIP: 0010:__alloc_pages_node include/linux/gfp.h:221 [inline] > > > > > > > RIP: 0010:hpage_collapse_alloc_page mm/khugepaged.c:807 [inline] > > > > > > > RIP: 0010:alloc_charge_hpage+0x802/0xaa0 mm/khugepaged.c:963 > > > > > > > Code: e5 01 4c 89 ee e8 6e f9 ae ff 4d 85 ed 0f 84 28 fc ff ff e8 70 fc ae ff 48 8d 6b ff 4c 8d 63 07 e9 16 fc ff ff e8 5e fc ae ff <0f> 0b e9 96 fa ff ff 41 bc 1a 00 00 00 e9 86 fd ff ff e8 47 fc ae > > > > > > > RSP: 0018:ffffc90003fdf7d8 EFLAGS: 00010293 > > > > > > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 > > > > > > > RDX: ffff888077f457c0 RSI: ffffffff81cd8f42 RDI: 0000000000000001 > > > > > > > RBP: ffff888079388c0c R08: 0000000000000001 R09: 0000000000000000 > > > > > > > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 > > > > > > > R13: dffffc0000000000 R14: 0000000000000000 R15: 0000000000000000 > > > > > > > FS: 00007f6b48ccf700(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000 > > > > > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > > > > > CR2: 00007f6b48a819f0 CR3: 00000000171e7000 CR4: 00000000003506e0 > > > > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > > > > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > > > > > > > Call Trace: > > > > > > > > > > > > > > collapse_file+0x1ca/0x5780 mm/khugepaged.c:1715 > > > > > > > > > > > > This is quite weird, isn't it? alloc_charge_hpage is selecting the most > > > > > > busy node (as per collapse_control). How come this can be an offline > > > > > > node? Is a parallel memory hotplug happening? > > > > > > > > > > TBH -- I did not look closely at the syzbot reproducer (let alone > > > > > attempt to run it) and assumed this was the case. Taking a quick look, > > > > > at least memory hot remove is enabled: > > > > > > > > > > CONFIG_ARCH_ENABLE_MEMORY_HOTPLUG=y > > > > > CONFIG_ARCH_ENABLE_MEMORY_HOTREMOVE=y > > > > > CONFIG_MEMORY_HOTPLUG=y > > > > > CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y > > > > > CONFIG_MEMORY_HOTREMOVE=y > > > > > > > > > > But looking at the C reproducer, I don't immediately see anywhere > > > > > where we offline nodes. I'll try to run this tomorrow to make sure I'm > > > > > not missing something real here. > > > > > > > > Looking slightly closer at hpage_collapse_scan_file I think that it is > > > > possible that xas_for_each simply doesn't find any entries in the page > > > > cache and with khugepaged_max_ptes_none == HPAGE_PMD_NR we can fall back > > > > to collapse_file even without any real entries. > > > > > > The khugepaged_max_ptes_none can't be HPAGE_PMD_NR, it must be <= > > > (HPAGE_PMD_NR - 1), but MADV_COLLAPSE does ignore it. > > > > > > But a closer look at the code about how to pick up the preferred node, > > > there seems to be a corner case for MADV_COLLAPSE. > > > > > > The code tried to do some balance if several nodes have the same hit > > > record. Basically it does conceptually: > > > * If the target_node <= last_target_node, then iterate from > > > last_target_node + 1 to MAX_NUMNODES (1024 on default config) > > > * If the max_value == node_load[nid], then target_node = nid > > > > > > So assuming the system has 2 nodes, the target_node is 0 and the > > > last_target_node is 1, if MADV_COLLAPSE path is hit, then it may > > > return 2 for target_node, but it is actually not existing (offline), > > > so the warn is triggered. > > > > > > > You're one step ahead of me, Yang. I was just debugging the syzbot C > > reproducer, and this seems to be exactly the case that is happening. > > > > > The below patch should be able to fix it: > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index ea0d186bc9d4..d24405e6736b 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > > > @@ -787,7 +787,8 @@ static int hpage_collapse_find_target_node(struct > > > collapse_control *cc) > > > if (target_node <= cc->last_target_node) > > > for (nid = cc->last_target_node + 1; nid < MAX_NUMNODES; > > > nid++) > > > - if (max_value == cc->node_load[nid]) { > > > + if (node_online(nid) && > > > + max_value == cc->node_load[nid]) { > > > target_node = nid; > > > break; > > > } > > > > > > > Thanks for the patch. I think this is the right place to do the check. > > > > This is slightly tangential - but I don't want to send a new mail > > about it -- but I wonder if we should be doing __GFP_THISNODE + > > explicit node vs having hpage_collapse_find_target_node() set a > > nodemask. We could then provide fallback nodes for ties, or if some > > node contained > some threshold number of pages. > > We definitely could, but I'm not sure whether we should make this code > more complicated or not. TBH I think it is already overengineered. If > fallback is fine, then just removing __GFP_THISNODE may be fine too. > Actually I doubt there would be any noticeable difference for real > life workload. > It would definitely make it more complicated. Fallback to certain nodes is ok -- we really would like to avoid allocating a THP if it's mostly accessed remotely. Anyways -- as you mention, likely no difference in real workloads, so fine leaving as-is. > > > > > > But the mere possibility of the hotplug race should be a sufficient > > > > ground to remove those WARN_ONs > > > > > > > Agreed. > > > > > The warn_on did help to catch this bug. But the reasons for removing > > > it still stand TBH, so we may consider to move this warn_on to the > > > callers which care about it? > > > > I didn't come across in a cursory search -- but if there are callers > > which try to synchronize with hot remove to ensure __GFP_THISNODE > > succeeds, then sure, the warn makes sense to them. > > Yeah, we are on the same page, the specific warning could be added by > the caller instead of in the common path. > > > > > > > > > > > > > > > Thanks! > > > > -- > > > > Michal Hocko > > > > SUSE Labs