Messages in this thread | | | Subject | Re: [PATCH v2 4.9 00/10] fix a race in release_task when flushing the dentry | From | Wen Yang <> | Date | Fri, 8 Jan 2021 10:42:47 +0800 |
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在 2021/1/8 上午2:28, Greg Kroah-Hartman 写道: > On Fri, Jan 08, 2021 at 12:21:38AM +0800, Wen Yang wrote: >> >> >> 在 2021/1/7 下午8:17, Greg Kroah-Hartman 写道: >>> On Thu, Jan 07, 2021 at 03:52:12PM +0800, Wen Yang wrote: >>>> The dentries such as /proc/<pid>/ns/ have the DCACHE_OP_DELETE flag, they >>>> should be deleted when the process exits. >>>> >>>> Suppose the following race appears: >>>> >>>> release_task dput >>>> -> proc_flush_task >>>> -> dentry->d_op->d_delete(dentry) >>>> -> __exit_signal >>>> -> dentry->d_lockref.count-- and return. >>>> >>>> In the proc_flush_task(), if another process is using this dentry, it will >>>> not be deleted. At the same time, in dput(), d_op->d_delete() can be executed >>>> before __exit_signal(pid has not been hashed), d_delete returns false, so >>>> this dentry still cannot be deleted. >>>> >>>> This dentry will always be cached (although its count is 0 and the >>>> DCACHE_OP_DELETE flag is set), its parent denry will also be cached too, and >>>> these dentries can only be deleted when drop_caches is manually triggered. >>>> >>>> This will result in wasted memory. What's more troublesome is that these >>>> dentries reference pid, according to the commit f333c700c610 ("pidns: Add a >>>> limit on the number of pid namespaces"), if the pid cannot be released, it >>>> may result in the inability to create a new pid_ns. >>>> >>>> This issue was introduced by 60347f6716aa ("pid namespaces: prepare >>>> proc_flust_task() to flush entries from multiple proc trees"), exposed by >>>> f333c700c610 ("pidns: Add a limit on the number of pid namespaces"), and then >>>> fixed by 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc"). >>> >>> Why are you just submitting a series for 4.9 and 4.19, what about 4.14? >>> We can't have users move to a newer kernel and then experience old bugs, >>> right? >>> >> Okay, the patches corresponding to 4.14 will be ready later. > > Note for some reason you didn't cc: the stable list for these patches :( > >>> But the larger question is why are you backporting a whole new feature >>> here? Why is CLONE_PIDFD needed? That feels really wrong... >>> >> >> The reason for backporting CLONE_PIDFD is because 7bc3e6e55acf ("proc: Use a >> list of inodes to flush from proc") relies on wait_pidfd.lock. There are >> indeed many associated modifications here. We are also testing it. Please >> check the code more. > > Is the only "issue" here wasted memory? Will it eventually be freed > anyway even if you do not echo to the proc file to flush caches? > > You mention the inability to create a new pid for a specific namespace, > is that really a problem? Shouldn't the code handle such issues > normally? What breaks without these changes? > > I think at this point, it might just time for you to move to a newer > kernel release, as adding a whole new userspace feature for this feels > really really odd. > > What is preventing you from doing that today? What holds you to older > kernels that will not allow you to move forward? >
We have encountered this problem in the cloud server environment. Users will frequently create and delete containers, and the corresponding pid_ns will accumulate, eventually making it impossible to create a new container.
https://bugzilla.kernel.org/show_bug.cgi?id=208613
The kernels (4.9/4.19) used on a large scale in our current production environment (almost tens of thousands of machines) may need to be fixed.
Thanks.
-- Best wishes, Wen
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