• Andy Lutomirski's avatar
    x86/mm/cpa: Fix populate_pgd(): Stop trying to deallocate failed PUDs · 530dd8d4
    Andy Lutomirski authored
    Valdis Kletnieks bisected a boot failure back to this recent commit:
    
      360cb4d1 ("x86/mm/cpa: In populate_pgd(), don't set the PGD entry until it's populated")
    
    I broke the case where a PUD table got allocated -- populate_pud()
    would wander off a pgd_none entry and get lost.  I'm not sure how
    this survived my testing.
    
    Fix the original issue in a much simpler way.  The problem
    was that, if we allocated a PUD table, failed to populate it, and
    freed it, another CPU could potentially keep using the PGD entry we
    installed (either by copying it via vmalloc_fault or by speculatively
    caching it).  There's a straightforward fix: simply leave the
    top-level entry in place if this happens.  This can't waste any
    significant amount of memory -- there are at most 256 entries like
    this systemwide and, as a practical matter, if we hit this failure
    path repeatedly, we're likely to reuse the same page anyway.
    
    For context, this is a reversion with this hunk added in:
    
    	if (ret < 0) {
    +		/*
    +		 * Leave the PUD page in place in case some other CPU or thread
    +		 * already found it, but remove any useless entries we just
    +		 * added to it.
    +		 */
    -		unmap_pgd_range(cpa->pgd, addr,
    +		unmap_pud_range(pgd_entry, addr,
    			        addr + (cpa->numpages << PAGE_SHIFT));
    		return ret;
    	}
    
    This effectively open-codes what the now-deleted unmap_pgd_range()
    function used to do except that unmap_pgd_range() used to try to
    free the page as well.
    Reported-by: default avatarValdis Kletnieks <Valdis.Kletnieks@vt.edu>
    Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Luis R. Rodriguez <mcgrof@suse.com>
    Cc: Mike Krinkin <krinkin.m.u@gmail.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Toshi Kani <toshi.kani@hp.com>
    Link: http://lkml.kernel.org/r/21cbc2822aa18aa812c0215f4231dbf5f65afa7f.1469249789.git.luto@kernel.orgSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    530dd8d4
pageattr.c 47.3 KB