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Message-ID: <CALCETrVCGeL6-Jji05CLrFUSyMOE6b5W2dqGNQoyUqLYXw0LLg@mail.gmail.com>
Date: Thu, 20 Dec 2012 21:42:46 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Jan Kara <jack@...e.cz>
Cc: linux-kernel@...r.kernel.org,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
Dave Chinner <david@...morbit.com>,
Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH 2/4] mm: Update file times when inodes are written
after mmaped writes
On Thu, Dec 20, 2012 at 4:34 PM, Jan Kara <jack@...e.cz> wrote:
> On Thu 20-12-12 15:10:10, Andy Lutomirski wrote:
>> The onus is currently on filesystems to call file_update_time
>> somewhere in the page_mkwrite path. This is unfortunate for three
>> reasons:
>>
>> 1. page_mkwrite on a locked page should be fast. ext4, for example,
>> often sleeps while dirtying inodes.
>>
>> 2. The current behavior is surprising -- the timestamp resulting from
>> an mmaped write will be before the write, not after. This contradicts
>> the mmap(2) manpage, which says:
>>
>> The st_ctime and st_mtime field for a file mapped with PROT_WRITE and
>> MAP_SHARED will be updated after a write to the mapped region, and
>> before a subsequent msync(2) with the MS_SYNC or MS_ASYNC flag, if one
>> occurs.
> I agree your behavior is more correct wrt to the manpage / spec. OTOH I
> could dig out several emails where users complain time stamps magically
> change some time after the file was written via mmap (because writeback
> happened at that time and it did some allocation to the inode). People hit
> this e.g. when compiling something, ld(1) writes final binary through mmap,
> the package / archive the final binary and later some sanity check finds
> the time stamp on the binary is newer than the package / archive.
>
> Looking more into the patch you end up updating timestamps on munmap(2)
> (thus on file close in particular). That should avoid the most surprising
> cases and users hopefully won't notice the difference. Good. But please
> mention this explicitely in the changelog.
I was careful to get that case right. I'll update the changelog.
In particular, I've so far tested munmap, msync(MS_SYNC), fsync,
waiting 30 seconds, and dying by fatal signal. All of those paths
work right.
>> +/**
>> + * inode_update_time_writable - update mtime and ctime time
>> + * @inode: inode accessed
>> + *
>> + * This is like file_update_time, but it assumes the mnt is writable
>> + * and takes an inode parameter instead.
>> + */
>> +
>> +int inode_update_time_writable(struct inode *inode)
>> +{
>> + struct timespec now;
>> + int sync_it = 0;
>> + int ret;
>> +
>> + /* First try to exhaust all avenues to not sync */
>> + if (IS_NOCMTIME(inode))
>> + return 0;
>> +
>> + now = current_fs_time(inode->i_sb);
>> + if (!timespec_equal(&inode->i_mtime, &now))
>> + sync_it = S_MTIME;
>> +
>> + if (!timespec_equal(&inode->i_ctime, &now))
>> + sync_it |= S_CTIME;
>> +
>> + if (IS_I_VERSION(inode))
>> + sync_it |= S_VERSION;
>> +
>> + if (!sync_it)
>> + return 0;
>> +
>> + ret = update_time(inode, &now, sync_it);
>> +
>> + return ret;
>> +}
>> +EXPORT_SYMBOL(inode_update_time_writable);
>> +
> So this differs from file_update_time() only by not calling
> __mnt_want_write(). Why this special function? It is actually unsafe wrt
> remounts read-only or filesystem freezing... For that you need to call
> sb_start_write() / sb_end_write() around the timestamp update. Umm, or
> better sb_start_pagefault() / sb_end_pagefault() because the call in
> remove_vma() gets called under mmap_sem so we are in a rather similar
> situation to ->page_mkwrite.
The important difference is that it takes an inode* as a parameter
instead of a file*. I don't think that inodes have a struct vfsmount,
so I can't call __mnt_want_write. I'll take a look at
sb_start_pagefault. I'll also refactor this a bit to minimize code
duplication. The current approach was for the v1 rfc version. :)
>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 3913262..60301dc 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -223,6 +223,10 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>> struct vm_area_struct *next = vma->vm_next;
>>
>> might_sleep();
>> +
>> + if (vma->vm_file)
>> + mapping_flush_cmtime(vma->vm_file->f_mapping);
>> +
>> if (vma->vm_ops && vma->vm_ops->close)
>> vma->vm_ops->close(vma);
>> if (vma->vm_file)
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index cdea11a..8cbb7fb 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1910,6 +1910,13 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
>> ret = mapping->a_ops->writepages(mapping, wbc);
>> else
>> ret = generic_writepages(mapping, wbc);
>> +
>> + /*
>> + * This is after writepages because the AS_CMTIME bit won't
>> + * bet set until writepages is called.
>> + */
>> + mapping_flush_cmtime(mapping);
>> +
>> return ret;
>> }
>>
>> @@ -2117,8 +2124,17 @@ EXPORT_SYMBOL(set_page_dirty);
>> */
>> int set_page_dirty_from_pte(struct page *page)
>> {
>> - /* Doesn't do anything interesting yet. */
>> - return set_page_dirty(page);
>> + int ret = set_page_dirty(page);
>> +
>> + /*
>> + * We may be out of memory and/or have various locks held, so
>> + * there isn't much we can do in here.
>> + */
>> + struct address_space *mapping = page_mapping(page);
> Declarations should go together please. So something like:
> int ret = set_page_dirty(page);
> struct address_space *mapping = page_mapping(page);
>
> /* comment... */
Will do. Some day I'll learn how to act less like a C99/C++
programmer when writing kernel code.
Am I correct in interpreting this as "these patches may be
sufficiently non-insane that I should keep working on them"? I admit
I'm pretty far out of my depth working on vm/vfs stuff.
Thanks,
Andy
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