On Fri, Feb 21, 2020 at 05:00:13PM -0800, Darrick J. Wong wrote: > On Thu, Feb 20, 2020 at 08:57:34AM -0800, Matthew Wilcox wrote: > > On Thu, Feb 20, 2020 at 07:49:12AM -0800, Christoph Hellwig wrote: > > +/** > > + * iomap_readahead - Attempt to read pages from a file. > > + * @rac: Describes the pages to be read. > > + * @ops: The operations vector for the filesystem. > > + * > > + * This function is for filesystems to call to implement their readahead > > + * address_space operation. > > + * > > + * Context: The file is pinned by the caller, and the pages to be read are > > + * all locked and have an elevated refcount. This function will unlock > > + * the pages (once I/O has completed on them, or I/O has been determined to > > + * not be necessary). It will also decrease the refcount once the pages > > + * have been submitted for I/O. After this point, the page may be removed > > + * from the page cache, and should not be referenced. > > + */ > > > > > Isn't the context documentation something that belongs into the aop > > > documentation? I've never really seen the value of duplicating this > > > information in method instances, as it is just bound to be out of date > > > rather sooner than later. > > > > I'm in two minds about it as well. There's definitely no value in > > providing kernel-doc for implementations of a common interface ... so > > rather than fixing the nilfs2 kernel-doc, I just deleted it. But this > > isn't just the implementation, like nilfs2_readahead() is, it's a library > > function for filesystems to call, so it deserves documentation. On the > > other hand, there's no real thought to this on the part of the filesystem; > > the implementation just calls this with the appropriate ops pointer. > > > > Then again, I kind of feel like we need more documentation of iomap to > > help filesystems convert to using it. But maybe kernel-doc isn't the > > mechanism to provide that. > > I think we need more documentation of the parts of iomap where it can > call back into the filesystem (looking at you, iomap_dio_ops). > > I'm not opposed to letting this comment stay, though I don't see it as > all that necessary since iomap_readahead implements a callout that's > documented in vfs.rst and is thus subject to all the constraints listed > in the (*readahead) documentation. Right. And that's not currently in kernel-doc format, but should be. Something for a different patchset, IMO. What we need documenting _here_ is the conditions under which the iomap_ops are called so the filesystem author doesn't need to piece them together from three different places. Here's what I currently have: * Context: The @ops callbacks may submit I/O (eg to read the addresses of * blocks from disc), and may wait for it. The caller may be trying to * access a different page, and so sleeping excessively should be avoided. * It may allocate memory, but should avoid large allocations. This * function is called with memalloc_nofs set, so allocations will not cause * the filesystem to be reentered.
