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List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/6] btrfs: use an on-stack path in btrfs_insert_orphan_item() To: dsterba@suse.cz, Jeff Layton Cc: Chris Mason , David Sterba , Qu Wenruo , linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com References: <20260811-btrfs-enomem-v3-0-46a993fc3fe5@kernel.org> <20260811-btrfs-enomem-v3-1-46a993fc3fe5@kernel.org> <20260820120428.GA9053@suse.cz> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNIlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT7CwJQEEwEIAD4CGwMFCwkI 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It is called from btrfs_orphan_add(), >> so a path allocation failure there turns a recoverable error into a >> transaction abort. >> >> btrfs_path is only ~112 bytes, so allocate it on the stack instead. >=20 > 112 is too much for on-stack, we've avoided that for btrfs_path in > particular, except some justified cases. This means in general the > beginning of call stack like ioctl, syscall handler and such. Otherwise > we assume there are other layers in the IO stack, like block device > drivers (DM), NFS, encoding layers or networking (iscsi), and obviously > the lowest level device drivers. I think you're very inconsistent on on-stack memory usage at least. You were fine when I was adding 128bytes for several call sites for the=20 support of huge pages, and I'd argue all those call sites have a deeper=20 stack, because it's on the writeback path. Furthermore, that huge page support is not widely used, but everyone=20 will need to pay that on-stack price. On the other hand, you are also very hesitant on my recent patches=20 removing those 128 bytes usages. So your behavior doesn't seem to match what you said here. Secondly, your deep-in-the-stack argument doesn't sound solid either. Every block file system can be built upon layer of storage stacks, not=20 only btrfs, but *every* block fs as long as there is a chance to do IO. This means you're just saying, there can be almost-infinite lower layers= =20 under us, so we should not use any extra on-stack memory. I do not think this is the sane nor really validated. If you want to argue if the extra 112 bytes is good or not, give me some= =20 data about the on-stack memory usage. With the proof that with enough stacked dm layer, that extra 112 bytes=20 are going to cause problem. Not to mention I believe some dm drivers are queuing the real submission= =20 handling into a workqueue, avoiding further increasing the on-stack=20 memory usage. >=20 > The trade off with possible allocation failure vs stack consumption > needs to be argued in the changelog, "is just 112" is not sufficient. Although I agree that more changelog will help, especially if Jeff has a= =20 good call trace showing that this is causing a flip RO in meta's fleet. >=20 > Getting back the consumed stack space is painful, we've been reducing > unneeded or redundant parameters of functions for years. The gains are > like -8 bytes here and -8 bytes there, allocation of +112 wipes that out= . >=20 > If the place of allocation is critical we can consider that but we have > too many of them, anywhere during the transaction commit path or > irreversible metadata changes. Possibly using __GFP_HIGH could work, but > I haven't explored that. >=20 > Qu added the patches to for-next but I had no chance to look closely at > this patchset yet and am hesitant to leave it like that. >=20