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. 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 -- 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
