On Wed, Feb 26, 2014 at 03:08:58PM -0800, Hugh Dickins wrote: > On Wed, 26 Feb 2014, Dave Chinner wrote: > > On Tue, Feb 25, 2014 at 08:45:15PM -0800, Hugh Dickins wrote: > > > On Wed, 26 Feb 2014, Dave Chinner wrote: > > > > On Tue, Feb 25, 2014 at 03:23:35PM -0800, Hugh Dickins wrote: > > > > > > > > > I should mention that when "we" implemented this thirty years ago, > > > > > we had a strong conviction that the system call should be idempotent: > > > > > that is, the len argument should indicate the final i_size, not the > > > > > amount being removed from it. Now, I don't remember the grounds for > > > > > that conviction: maybe it was just an idealistic preference for how > > > > > to design a good system call. I can certainly see that defining it > > > > > that way round would surprise many app programmers. Just mentioning > > > > > this in case anyone on these lists sees a practical advantage to > > > > > doing it that way instead. > > > > > > > > I don't see how specifying the end file size as an improvement. What > > > > happens if you are collapse a range in a file that is still being > > > > appended to by the application and so you race with a file size > > > > update? IOWs, with such an API the range to be collapsed is > > > > completely unpredictable, and IMO that's a fundamentally broken API. > > > > > > That's fine if you don't see the idempotent API as an improvement, > > > I just wanted to put it on the table in case someone does see an > > > advantage to it. But I think I'm missing something in your race > > > example: I don't see a difference between the two APIs there. > > > > > > Userspace can't sample the inode size via stat(2) and then use the value for a > > syscall atomically. i.e. if you specify the offset you want to > > collapse at, and the file size you want to have to define the region > > to collapse, then the length you need to collapse is (current inode > > size - end file size). If "current inode size" can change between > > the stat(2) and fallocate() call (and it can), then the length being > > collapsed is indeterminate.... > > Thanks for explaining more, I was just about to acknowledge what a good > example that is. Indeed, it seems not unreasonable to be editing the > earlier part of a file while the later part of it is still streaming in. > > But damn, it now occurs to me that there's still a problem at the > streaming end: its file write offset won't be updated to reflect > the collapse, so there would be a sparse hole at that end. And > collapse returns -EPERM if IS_APPEND(inode). Well, we figure that most applications won't be using append only inode flags for files that they know they want to edit at random offsets later on. ;) However, I can see how DVR apps would use open(O_APPEND) to obtain the fd they write to because that sets the write position to the EOF on every write() call (i.e. in generic_write_checks()). And collapse range should behave sanely with this sort of usage. e.g. XFS calls generic_write_checks() after it has taken the IO lock to set the current write position to EOF. Hence it will be correctly serialised against collapse range calls and so O_APPEND writes will not leave sparse holes if collapse range calls are interleaved with the write stream.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
