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x86/mm/cpa: Fix populate_pgd(): Stop trying to deallocate failed PUDs

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Valdis Kletnieks bisected a boot failure back to this recent commit:

  360cb4d155 ("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: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Andy 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.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-07-22 21:58:08 -07:00 committed by Ingo Molnar
parent 3ebfd81f7f
commit 530dd8d4b9
1 changed files with 2 additions and 5 deletions
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7
arch/x86/mm/pageattr.c
View File

@ -1082,6 +1082,8 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
pud = (pud_t *)get_zeroed_page(GFP_KERNEL | __GFP_NOTRACK);
if (!pud)
return -1;
set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));
}
pgprot_val(pgprot) &= ~pgprot_val(cpa->mask_clr);
@ -1089,16 +1091,11 @@ static int populate_pgd(struct cpa_data *cpa, unsigned long addr)
ret = populate_pud(cpa, addr, pgd_entry, pgprot);
if (ret < 0) {
if (pud)
free_page((unsigned long)pud);
unmap_pud_range(pgd_entry, addr,
addr + (cpa->numpages << PAGE_SHIFT));
return ret;
}
if (pud)
set_pgd(pgd_entry, __pgd(__pa(pud) | _KERNPG_TABLE));
cpa->numpages = ret;
return 0;
}