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Subject: Re: [PATCH v2 5/6] proc: use down_read_killable mmap_sem for
 /proc/pid/map_files
To:     Andrei Vagin <avagin@gmail.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Roman Gushchin <guro@fb.com>, Dmitry Safonov <dima@arista.com>
References: <156007465229.3335.10259979070641486905.stgit@buzz>
 <156007493995.3335.9595044802115356911.stgit@buzz>
 <20190612231426.GA3639@gmail.com>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <f15478b5-098f-e1be-0928-62f46cff77e7@yandex-team.ru>
Date:   Thu, 13 Jun 2019 11:15:50 +0300
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On 13.06.2019 2:14, Andrei Vagin wrote:
> On Sun, Jun 09, 2019 at 01:09:00PM +0300, Konstantin Khlebnikov wrote:
>> Do not stuck forever if something wrong.
>> Killable lock allows to cleanup stuck tasks and simplifies investigation.
> 
> This patch breaks the CRIU project, because stat() returns EINTR instead
> of ENOENT:
> 
> [root@fc24 criu]# stat /proc/self/map_files/0-0
> stat: cannot stat '/proc/self/map_files/0-0': Interrupted system call

Good catch.

It seems CRIU tests has good coverage for darkest corners of kernel API.
Kernel CI projects should use it. I suppose you know how to promote this. =)

> 
> Here is one inline comment with the fix for this issue.
> 
>>
>> It seems ->d_revalidate() could return any error (except ECHILD) to
>> abort validation and pass error as result of lookup sequence.
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>> Reviewed-by: Roman Gushchin <guro@fb.com>
>> Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
>> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> 
> It was nice to see all four of you in one place :).
> 
>> Acked-by: Michal Hocko <mhocko@suse.com>
>> ---
>>   fs/proc/base.c |   27 +++++++++++++++++++++------
>>   1 file changed, 21 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 9c8ca6cd3ce4..515ab29c2adf 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -1962,9 +1962,12 @@ static int map_files_d_revalidate(struct dentry *dentry, unsigned int flags)
>>   		goto out;
>>   
>>   	if (!dname_to_vma_addr(dentry, &vm_start, &vm_end)) {
>> -		down_read(&mm->mmap_sem);
>> -		exact_vma_exists = !!find_exact_vma(mm, vm_start, vm_end);
>> -		up_read(&mm->mmap_sem);
>> +		status = down_read_killable(&mm->mmap_sem);
>> +		if (!status) {
>> +			exact_vma_exists = !!find_exact_vma(mm, vm_start,
>> +							    vm_end);
>> +			up_read(&mm->mmap_sem);
>> +		}
>>   	}
>>   
>>   	mmput(mm);
>> @@ -2010,8 +2013,11 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
>>   	if (rc)
>>   		goto out_mmput;
>>   
>> +	rc = down_read_killable(&mm->mmap_sem);
>> +	if (rc)
>> +		goto out_mmput;
>> +
>>   	rc = -ENOENT;
>> -	down_read(&mm->mmap_sem);
>>   	vma = find_exact_vma(mm, vm_start, vm_end);
>>   	if (vma && vma->vm_file) {
>>   		*path = vma->vm_file->f_path;
>> @@ -2107,7 +2113,10 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
>>   	if (!mm)
>>   		goto out_put_task;
>>   
>> -	down_read(&mm->mmap_sem);
>> +	result = ERR_PTR(-EINTR);
>> +	if (down_read_killable(&mm->mmap_sem))
>> +		goto out_put_mm;
>> +
> 
> 	result = ERR_PTR(-ENOENT);
> 
>>   	vma = find_exact_vma(mm, vm_start, vm_end);
>>   	if (!vma)
>>   		goto out_no_vma;
>> @@ -2118,6 +2127,7 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
>>   
>>   out_no_vma:
>>   	up_read(&mm->mmap_sem);
>> +out_put_mm:
>>   	mmput(mm);
>>   out_put_task:
>>   	put_task_struct(task);
>> @@ -2160,7 +2170,12 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
>>   	mm = get_task_mm(task);
>>   	if (!mm)
>>   		goto out_put_task;
>> -	down_read(&mm->mmap_sem);
>> +
>> +	ret = down_read_killable(&mm->mmap_sem);
>> +	if (ret) {
>> +		mmput(mm);
>> +		goto out_put_task;
>> +	}
>>   
>>   	nr_files = 0;
>>