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    revert "procfs: provide stack information for threads" and its fixup commits · 34441427
    Robin Holt authored
    Originally, commit d899bf7b ("procfs: provide stack information for
    threads") attempted to introduce a new feature for showing where the
    threadstack was located and how many pages are being utilized by the
    stack.
    
    Commit c44972f1 ("procfs: disable per-task stack usage on NOMMU") was
    applied to fix the NO_MMU case.
    
    Commit 89240ba0 ("x86, fs: Fix x86 procfs stack information for threads on
    64-bit") was applied to fix a bug in ia32 executables being loaded.
    
    Commit 9ebd4eba ("procfs: fix /proc/<pid>/stat stack pointer for kernel
    threads") was applied to fix a bug which had kernel threads printing a
    userland stack address.
    
    Commit 1306d603 ('proc: partially revert "procfs: provide stack
    information for threads"') was then applied to revert the stack pages
    being used to solve a significant performance regression.
    
    This patch nearly undoes the effect of all these patches.
    
    The reason for reverting these is it provides an unusable value in
    field 28.  For x86_64, a fork will result in the task->stack_start
    value being updated to the current user top of stack and not the stack
    start address.  This unpredictability of the stack_start value makes
    it worthless.  That includes the intended use of showing how much stack
    space a thread has.
    
    Other architectures will get different values.  As an example, ia64
    gets 0.  The do_fork() and copy_process() functions appear to treat the
    stack_start and stack_size parameters as architecture specific.
    
    I only partially reverted c44972f1 ("procfs: disable per-task stack usage
    on NOMMU") .  If I had completely reverted it, I would have had to change
    mm/Makefile only build pagewalk.o when CONFIG_PROC_PAGE_MONITOR is
    configured.  Since I could not test the builds without significant effort,
    I decided to not change mm/Makefile.
    
    I only partially reverted 89240ba0
    
     ("x86, fs: Fix x86 procfs stack
    information for threads on 64-bit") .  I left the KSTK_ESP() change in
    place as that seemed worthwhile.
    
    Signed-off-by: default avatarRobin Holt <holt@sgi.com>
    Cc: Stefani Seibold <stefani@seibold.net>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: Michal Simek <monstr@monstr.eu>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: <stable@kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    34441427