On 11/17/2017 12:45 PM, Nikolay Borisov wrote: > > > On 17.11.2017 19:44, Goldwyn Rodrigues wrote: >> This patch series attempts to use kernels iomap for btrfs. Currently, >> it covers buffered writes only, but I intend to add some other iomap >> uses once this gets through. I am sending this as an RFC because I >> would like to find ways to improve the solution since some changes >> require adding more functions to the iomap infrastructure which I >> would try to avoid. I still have to remove some kinks as well such >> as -o compress. I have posted some questions in the individual >> patches and would appreciate some input to those. >> >> Some of the problems I faced is: >> >> 1. extent locking: While we perform the extent locking for writes, >> we need to perform any reads because of non-page-aligned calls before >> locking can be done. This requires reading the page, increasing their >> pagecount and "letting it go". The iomap infrastructure uses >> buffer_heads wheras btrfs uses bio and hence needs to call readpage >> exclusively. The "letting it go" part makes me somewhat nervous of >> conflicting reads/writes, even though we are protected under i_rwsem. >> Is readpage_nolock() a good idea? The extent locking sequence is a >> bit weird, with locks and unlock happening in different functions. > > Is there some inherent requirement in iomap's design that necessitates > the usage of buffer heads? I thought the trend is for buffer_head to > eventually die out. Given that iomap is fairly recent (2-3 years?) I > find it odd it's relying on buffer heads. >
No, there is no inherent reason that I see, but legacy. iomap is carved out of existing filesystems such as xfs which traditionally use buffer_heads. In any case, the buffer heads make I/O to individual pages independently. iomap calls existing functions which use buffer heads. >> >> 2. btrfs pages use PagePrivate to store EXTENT_PAGE_PRIVATE which is not >> used anywhere. >> However, a PagePrivate flag is used for try_to_release_buffers(). Can >> we do away with PagePrivate for data pages? The same with PageChecked. >> How and why is it used (I guess -o compress) >> >> 3. I had to stick information which will be required from iomap_begin() >> to iomap_end() in btrfs_iomap which is a pointer in btrfs_inode. Is >> there any other place/way we can transmit this information. XFS only >> performs allocations and deallocations so it just relies of bmap code >> for it. >> >> Suggestions/Criticism welcome. >> -- Goldwyn -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
