Re: Problems with determining data presence by examining extents?

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Tue, Jan 14, 2020 at 04:48:29PM +0000, David Howells wrote:
> Again with regard to my rewrite of fscache and cachefiles:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter
> 
> I've got rid of my use of bmap()!  Hooray!
> 
> However, I'm informed that I can't trust the extent map of a backing file to
> tell me accurately whether content exists in a file because:
> 
>  (a) Not-quite-contiguous extents may be joined by insertion of blocks of
>      zeros by the filesystem optimising itself.  This would give me a false
>      positive when trying to detect the presence of data.
> 
>  (b) Blocks of zeros that I write into the file may get punched out by
>      filesystem optimisation since a read back would be expected to read zeros
>      there anyway, provided it's below the EOF.  This would give me a false
>      negative.

The whole idea of an out of band interface is going to be racy and suffer
from implementation loss.  I think what you want is something similar to
the NFSv4.2 READ_PLUS operation - give me that if there is any and
otherwise tell me that there is a hole.  I think this could be a new
RWF_NOHOLE or similar flag, just how to return the hole size would be
a little awkward.  Maybe return a specific negative error code (ENODATA?)
and advance the iov anyway.



[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]

  Powered by Linux