Hi Alex, I think your question is about why workdir was introduced. I'd like to share my thought with you. I should say Miklos is the best candidate to explain it, because he is the author of this filesystem. On 2015/1/16 1:09, A. Wan wrote: > Just one question. If each r/o layer does not require a workdir, why > would a stack of r/o layers require one - and hence the requirement that > the top layer must be r/w? > As far as I know, "workdir" is not belong to any r/o or r/w layer in overlayfs. Actually it belong to a overlayfs mount itself. "Workdir" is not mandatory after recently multi-lower layer feature. It is used only in r/w case, and only valuable in r/w case. My patch wants to make sure the specified "workdir" in option line is significative. If you just mount a stack of r/o layers, you don't need any "workdir", but the mounted overlayfs partition will be marked as r/o. If you want a r/w mount, you must specify a upperdir, the r/w layer on the top and specify a workdir in the same mount with upperdir. Overlayfs is a union filesystem. It gives ability to combine directories in different mounts together. For r/w mount case, overlayfs allow users change any file in its mount. But the implement of overlayfs not directly write to each lower fs. All the write is reflected on the upper layer, the top r/w layer. See this link about unionfs: http://lwn.net/Articles/324291/ "Workdir" works as the temp directory of a overlayfs mount. The file changing is first done in it and then use rename() move to upper directory. That's why "workdir" must in the same mount with upperdir. For example, if users delete a file which belong to a lower a/o layer in overlayfs partition, the deletion is not perform on lower fs. Instead, a whiteout file is created in "workdir" and then move to upperdir. Other operations, like "copy_up", "rename" also use "workdir" as the temporary directory. You can read this code in fs/overlayfs yourself. I think "workdir" is used to keep the consistency of overlayfs and avoid corrupt data damaging the filesystem. I'd like Miklos could explain more for us. > Does it has to do with why workdir was introduced in the first place? It's not the business of my patch, "workdir" is not introduced by it. But after rescanning the document in kernel, I think more explanation about "workdir" is worth to be done. > Sorry but I couldn't find information about why workdir was introduced. I > suppose it was to support some functions that older versions can't. > No, "workdir" is introduced since overlayfs first merged into mainline and is still needed in r/w mount. But I wonder if there are other implements instead. Oh, Miklos may be unhappy with my unorthodox notion. I'm sorry to say I'm not good at English expression, but I'm happy to clarify any points that are still unclear. Thanks, Hu > Alex > > On Thu, January 15, 2015 12:20 am, Seunghun Lee wrote: >> On January 15, 2015 2:20:57 PM GMT+09:00, hujianyang >> <hujianyang@xxxxxxxxxx> wrote: >>> After importing multi-lower layer support, users could mount a r/o >>> partition as the left most lowerdir instead of using it as upperdir. >>> And a r/o upperdir may cause an error like >>> >>> overlayfs: failed to create directory ./workdir/work >>> >>> during mount. -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
