On Thu, Jun 12, 2014 at 09:37:58PM -0700, JP Abgrall wrote: > On Thu, Jun 12, 2014 at 8:30 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Fri, Jun 13, 2014 at 01:15:38PM +1000, Dave Chinner wrote: > >> Indeed, mixing -o discard and SFITRIM is a recipe for > >> confusion and leakage - "but I used secure trim on the device!" - > >> and so all discards either have to be secure or not. > > The idea was to keep on not using -o discard. And move from FITRIM to SFITRIM. IOWs, you want either normal discard or secure discard, and not a mix of both. IOWs, you don't need SFITRIM, you need a "block device does secure discard only" configuration flag... > > Oh, and while I think of it secure discard at the filesystem level > > isn't even a guarantee that you'll get rid of all stale references > > to a sector - if the filesystem has freed and then re-allocated a > > block without having gone through a discard cycle on that block, > > then the underlying device may have old copies of the block that it > > hasn't garbage collected and SFITRIM won't clean those up because it > > won't ask to trim in-use blocks.... > > Arg. So, if understand this correctly, if the eMMC chip won't get a > secure discard/trim of a block that gets reassigned to the FS, then ^^ within > data duplicates within the eMMC related to that block are not cleared, > and the next SFITRIM won't even reach that block or the duplicates as > the FS says they are in use. Pretty much. And even using -o discard is no guarantee that the filesystem will issue a discard between freeing and re-using a block e.g. XFS explicitly avoids issuing discards for blocks it re-uses immediately because they are always considered "in-use" from a transactional POV. Hence there is no place where the block is considered free, and hence there isn't a point in time where a discard can be safely issued on that block. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
