בתאריך יום ה׳, 7 בנוב׳ 2024 ב-7:03 מאת Theodore Ts'o <tytso@xxxxxxx>: > > On Wed, Oct 16, 2024 at 02:16:24PM +0300, Max Brener wrote: > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219306 > > v1: https://lore.kernel.org/lkml/20240926221103.24423-1-linmaxi@xxxxxxxxx/T/ > > > > Changes from last version: Moved vfs-level changes to be ext4-level, > > and improved the description of the patch. > > > > This patch enables skipping no-op 'ext4_truncate' calls. Analyzing the kernel > > with ftrace shows ext4_truncate is being sometimes called without making any > > impact, and sometimes userspace programs might call ext4_truncate in vein. By > > detecting these calls and skipping them, cpu time is saved. > > > > I'll fix this by skipping ext4_truncate call in 'ext4_setattr' when the file's size > > hasn't changed AND it hasn't been truncated since the last disk space preallocation. > > It is meant to consider the case when ext4_truncate is being called to truncate > > preallocated blocks too. Notice that so far, the condition to triggering > > ext4_truncate by the user was: if (attr->ia_size <= oldsize) which means it is > > being triggered when attr->ia_size == oldsize regardless of whether there are > > preallocated blocks or not - if there are none, then the call is redundant. > > > > Steps: > > 1.Add a new inode state flag: EXT4_STATE_TRUNCATED > > 2.Clear the flag when ext4_fallocate is being called with FALLOC_FL_KEEP_SIZE flag > > to enable using ext4_truncate again, to remove preallocated disk space that may > > have resulted from this call. > > 3.Set EXT4_STATE_TRUNCATED when ext4_truncated is called successfully. > > 4.Don't skip ext4_truncate in ext4_setattr when the size of the file has either been > > reduced OR stayed the same, but hasn't been truncated yet. This is in order to allow > > truncating of preallocated blocks. > > This patch is still not quite right. See Jan's comment from [1]: > > Agreed as well. I'll also note that keeping such flag uptodate is not as > simple as it seems because there are various places that may be allocating > blocks beyond EOF (for example extending writes) and that rely on > ext4_truncate() removing them so one needs to be careful to capture all the > places where the "truncated" state needs to be cleared. > > [1] https://lore.kernel.org/all/20240930095601.x66iqw74bxffytgq@quack3/ > > - Ted Okay I understand now, I initially thought any preallocation is necessarily done through a VFS interface. Now that I see preallocations are done at mballoc, what I can offer is to clear the TRUNCATED flag at ext4_mb_new_blocks(). Would that be ok in your opinion?
