On Sat, Aug 27, 2011 at 5:04 PM, Yongqiang Yang <xiaoqiangnk@xxxxxxxxx> wrote: > On Sat, Aug 27, 2011 at 6:35 AM, Allison Henderson > <achender@xxxxxxxxxxxxxxxxxx> wrote: >> On 08/25/2011 07:53 PM, Yongqiang Yang wrote: >>> >>> Hi Allison, >>> >>> Currently, punch hole flushes all pages to disk and releases pages in >>> page cache, and then calls ext4_ext_map_blocks. >>> >>> Assume that if a new page in the punching's range is mapped after >>> releasing pages and before down_write i_data_sem, >>> then ext4_ext_map_blocks will release map info of the page in extent >>> tree. However, up layers does not know this, and they think the page >>> is mapped. >>> >>> I can not find how punch hole handle the situation above. Could you >>> shed a light on it? >>> >>> >> Hi Yongqiang >> >> This is a really good question and at the moment Im still looking into it. >> :) The calling sequence in punch hole was modeled after truncate, which >> also only locks i_data_sem when modifying the extent tree. >> ext4_ext_map_blocks when called with the punch hole flag, only releases >> blocks in the extent tree, using the same routines truncate does, but it >> does not modify the state of the pages. Though that still does not prevent >> the race condition you describe, so I am still investigating it. >> I've found that I can catch a lot of race conditions by simply running the >> stress test over night, and so far I havnt had anything like this come up, >> but that certainly doesnt mean its not there. I will let you know what I >> find. Thx! > > Hi Allison, > > I had a look at truncate code, truncates and writes are serialized by > inode->i_mutex in vfs layer, but fallocate does not take i_mutex, so > we need to take i_mutex in punching hole as well, I think. Fallocate > behaves differently with punching hole, so it is safe without taking > i_mutex. It seems that race exists between reads and punching hole as well. If a read comes after releasing pages and before down_write(i_data_sem), then a page will be mapped, if the page is written later, it will introduce an error. truncate avoids this situation by set file size before truncating pages. Yongqiang. > > > What's your opinion? > > Yongqiang. >> >> Allison Henderson >> -- >> 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 >> > > > > -- > Best Wishes > Yongqiang Yang > -- Best Wishes Yongqiang Yang -- 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
