On Tue 04-09-12 16:11:32, Ian Abbott wrote: > 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). I see. Actually I thought linux-fsdevel is inherited from the default of fs/ directory. But now I tried get_maintainer.pl script and I can see that it's not. I'll update 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. OK, that sounds reasonably. > >>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. But don't you fall back to buffered IO for files in ICB? So then no zeroing should happen? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
