On Mon, Jul 22, 2013 at 10:17:42AM +0800, Zheng Liu wrote: > On Mon, Jul 22, 2013 at 11:38:31AM +1000, Dave Chinner wrote: > > On Fri, Jul 19, 2013 at 12:19:30PM -0400, Theodore Ts'o wrote: > > > On Fri, Jul 19, 2013 at 01:33:09PM +1000, Dave Chinner wrote: > > > > An ioctl is kinda silly for this. Just use O_NONBLOCK when calling > > > > open() and do the prefetch right in the open call. The open() can > > > > block, anyway, and what you are trying to do is non-blocking IO with > > > > AIO, so it seems like we've already got a sensible, generic > > > > interface for triggering this sort of prefetch operation. > > > > > > O_NONBLOCK (either set via open or fcntl) is a possibility, since it's > > > carefully defined to be unspecified for regular files by SUSv3. It is > > > quite different from the existing semantics for O_NONBLOCK, though. > > > Currently, for all file types where O_NONBLOCK is not ignored, open(2) > > > is guaranteed itself not to block. If we use O_NONBLOCK for regular > > > files to mean that any necessary metadata blocks required for AIO to > > > be "A" will be cached, then it will make open(2) much more likely to > > > block. Also, for all file types where O_NONBLOCK is not ignored, > > > read(2) will not block but instead return -1 and set errno to EAGAIN. > > > This would also be a change. > > > > > > If we tried to get this new semantics for O_NONBLOCK to be accepted by > > > the Austin Group for standardization in the future, would they accept > > > it, or would they say, "this makes me vommit"? I have a suspicion > > > there reaction might be closer to the latter.... > > > > > > If we want a VFS-level API, in my opinion an fadvise() flag would be a > > > better choice. > > > > Sure. Make it an fadvise() flag - just don't add ioctls for things > > that are generically useful. > > > > On second thoughts - you're trying to get the extent map read in. We > > already have an interface for querying extent maps - fiemap. > > FIEMAP_FLAG_PREFETCH along with the range of the file you want the > > extent map prefetched for? > > I don't think fiemap is a good interface. The application uses > fiemap(2) to retrieve extent mapping. fiemap is used to query information about extent maps. What it returns is entirely dependent on the input parameters that are passed to it. Indeed, from Documentation/filesystems/fiemap.txt: "If fm_extent_count is zero, then the fm_extents[] array is ignored (no extents will be returned), and the fm_mapped_extents count will hold the number of extents needed in fm_extents[] to hold the file's current mapping." Think about that for a minute. What does the filesystem do with such an fiemap query when the extent map is not cached? That's right, *fiemap reads the extent map from disk into the cache* and then returns the number of extents in the range. All I have suggested is adding a flag to make this an *explicit operation* rather than a side effect of a "count extents" query. I fail to see any justification for a whole new interface when we already have a perfectly functional one that already provides the functionality that is required... > That means that the app could use > these mappings in userspace. But now we want to cache these mappings in > kernel space. If the filesystem is not caching the extents read in during fiemap operations then perhaps you should look into fixing that deficiency. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
