On 2012-09-04 15:39, Jan Kara wrote:
Hello,
first, you have my address wrong (you had suze instead of suse) which is
why I wasn't getting your email and not replying (missed the patch in LKML
traffic). Second, it's good to CC also linux-fsdevel for UDF related
matters (I tend to use that for UDF announcements etc. so people caring
about UDF can watch there and don't have to read high-volume LKML).
Oops, sorry about the misspelling. Also, I've noted the linux-fsdevel
for future (I was just following what it said in MAINTAINERS).
On Tue 04-09-12 10:49:39, Ian Abbott wrote:
Add support for the O_DIRECT flag. There are two cases to deal with:
Out of curiosity, do you have a use for this feature or is it mostly
academic interest?
I'm planning to use it for an embedded project that needs to stream
large files off a CompactFlash card, but the data doesn't need to be in
the buffer cache as its only read once, and the system has very limited
memory bandwidth so I can't afford the the extra copy. The old version
of this project only supported FAT, but that limited the file size to
about 4GiB. The filesystem needs to be something reasonably
Windows-friendly, at least for adding the files to the CompactFlash card
in the first place.
1. Small files stored in the ICB (inode control block?): just return 0
from the new udf_adinicb_direct_IO() handler to fall back to buffered
I/O. For direct writes, there is a "gotcha" to deal with when
generic_file_direct_write() in mm/filemap.c invalidates the pages. In
the udf_adinicb_writepage() handler, only part of the page data will be
valid and the rest will be zeroed out, so only copy the valid part into
the ICB. (This is actually a bit inefficient as udf_adinicb_write_end()
will have already copied the data into the ICB once, but it's pretty
likely that the file will grow to the point where its data can no longer
be stored in the ICB and will be moved to a different area of the file
system. At that point, a different direct_IO handler will be used - see
below.)
Sorry, I didn't quite get this. What is the problem with copying all the
data to inode in udf_adinicb_writepage() as it is now?
Part of the good data in the ICB outside the range being addressed would
get overwritten by zeroes. This can be tested by creating a UDF
filesystem with 4KiB blocks and with small files stored in the ICB,
backed by a block device with 512 byte sectors. Create a 2KiB file with
random (or non-zero) data on the file system so that its data gets
stored in the ICB. Then open the file for writing without truncation
and with the O_DIRECT flag set, write 512 bytes at some 512 byte offset
within the 2KiB file and close it. If you then hexdump the file, you'll
find some of the old random data has been zeroed out.
--
-=( Ian Abbott @ MEV Ltd. E-mail: <abbotti@xxxxxxxxx> )=-
-=( Tel: +44 (0)161 477 1898 FAX: +44 (0)161 718 3587 )=-
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