How to safely access inode/dentry obtained from struct page *?

[Rohan Puri]

On Thu, Sep 4, 2014 at 1:33 AM, Rohan Puri <rohan.puri15 at gmail.com> wrote:
> On Wed, Sep 3, 2014 at 10:50 PM, Joshi <joshiiitr at gmail.com> wrote:
>>
>> Thanks Pranay. Please see below-
>>
>> On Mon, Sep 1, 2014 at 10:19 AM, Pranay Srivastava <pranjas at gmail.com> wrote:
>> >
>> > On 01-Sep-2014 10:18 AM, "Pranay Srivastava" <pranjas at gmail.com> wrote:
>> >>
>> >>
>> >> On 30-Aug-2014 10:49 AM, "Joshi" <joshiiitr at gmail.com> wrote:
>> >> >
>> >> > I am trying to obtain file name at block layer level (above IO
>> >> > scheduler).
>> >> > At this level I receive bio structure, within which page pointers are
>> >> > kept.
>> >> >
>> >> > Thereby I do following to obtain the inode and dentry -
>> >> >
>> >> > struct inode *inode_ptr = page->mapping->host;
>> >> > if (inode_ptr != NULL)
>> >> > /* Access dentry i.e. i_dentry in the inode */
>> >> >
>> >>
>> >> Are you doing this in readpage(s) or writepage(s) callback? If that's the
>> >> case your page would be locked and dentry/inode wouldn't go away.
>> >>
>> >> If you are doing something else then first make sure you do lock_page and
>> >> proceed only if you get that page lock.
>>
>> I am not operating at file-system level. I am operating on the bio
>> that file-system(or any other component above IO scheduler) might have
>> sent.
>> Do you think that page descriptor kept in bio (for write) is going to
>> vanish? I am not accessing the data of the page, just the
>> page->mapping pointer.

 Yes, it can happen. Race condition for your case can happen either due
 to page mapping removed or dentry removed from cache.Cases that I can
 think of are invalidations/truncation of page mapping happening
 (truncate, file delete, file hole punching) & as default IO is async
 so dentry can be evicted due to load as in fsstress by the time you
 print filename.

>>
>>
>> >> Second you can try dget and dput before you start working with dentry.
>>
>> I will try that.
>>
>> >> > This works usually. But problem is, inode and dentry may get released
>> >> > from inode and dentry cache at any time.
>> >> > While accessing inode/dentry I need to ensure that till the time I am
>> >> > accessing'em these structure remain valid in memory.
>> >> > Is it possible to ensure that? Which structures/locks I need to check
>> >> > for the purpose.
>> >>
>> >> is this your observation from your test case? Can you explain your test
>> >> case a bit.
>>
>> No, normally I obtain file name. But with fsstress, this causes crash.
>>
>> >>
>> > Have you seen d_alias and see if that can help you with filename?
>>
>> I do get filename from dentry. But while I am accessing that, it can
>> be freed by vfs.
>>
>> >
>> >> >
>> >> > Appreciate any help.
>> >> >
>> >> > Thanks!
>> >> >
>> >> > --
>> >> > Joshi
>> >> >
>> >> > _______________________________________________
>> >> > Kernelnewbies mailing list
>> >> > Kernelnewbies at kernelnewbies.org
>> >> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>>
>>
>> --
>> Joshi
>>
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>
 Make use of proper synchronization stuff as Pranay pointed out. Btw,
 what is the use case you need this for ? (just curious to know)

 - Rohan