Re: [patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Fri, Apr 15, 2022 at 03:10:51PM -0700, Linus Torvalds wrote:
> On Thu, Apr 14, 2022 at 7:13 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Revert shmem_file_read_iter() to using ZERO_PAGE for holes only when
> > iter_is_iovec(); in other cases, use the more natural iov_iter_zero()
> > instead of copy_page_to_iter().  We would use iov_iter_zero() throughout,
> > but the x86 clear_user() is not nearly so well optimized as copy to user
> > (dd of 1T sparse tmpfs file takes 57 seconds rather than 44 seconds).
> 
> Ugh.
> 
> I've applied this patch, but honestly, the proper course of action
> should just be to improve on clear_user().
> 
> If it really is important enough that we should care about that
> performance, then we just should fix clear_user().
> 
> It's a very odd special thing right now (at least on x86-64) using
> some strange handcrafted inline asm code.
> 
> I assume that 'rep stosb' is the fastest way to clear things on modern
> CPU's that have FSRM, and then we have the usual fallbacks (ie ERMS ->
> "rep stos" except for small areas, and probably that "store zeros by
> hand" for older CPUs).
> 
> Adding PeterZ and Borislav (who seem to be the last ones to have
> worked on the copy and clear_page stuff respectively) and the x86
> maintainers in case somebody gets the urge to just fix this.

Perhaps the x86 maintainers would like to start from
https://lore.kernel.org/lkml/20210523180423.108087-1-sneves@xxxxxxxxx/
instead of pushing that work off on the submitter.



[Index of Archives]     [Kernel Archive]     [IETF Annouce]     [DCCP]     [Netdev]     [Networking]     [Security]     [Bugtraq]     [Yosemite]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux SCSI]

  Powered by Linux