| Message ID | alpine.DEB.2.21.2001171411020.56385@chino.kir.corp.google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] mm, thp: fix defrag setting if newline is not used | expand |
On 1/17/20 11:11 PM, David Rientjes wrote: > If thp defrag setting "defer" is used and a newline is *not* used when > writing to the sysfs file, this is interpreted as the "defer+madvise" > option. > > This is because we do prefix matching and if five characters are written > without a newline, the current code ends up comparing to the first five > bytes of the "defer+madvise" option and using that instead. > > Use the more appropriate sysfs_streq() that handles the trailing newline > for us. Since this doubles as a nice cleanup, do it in enabled_store() > as well. > > Fixes: 21440d7eb904 ("mm, thp: add new defer+madvise defrag option") > Cc: Vlastimil Babka <vbabka@suse.cz> > Cc: Mel Gorman <mgorman@techsingularity.net> > Suggested-by: Andrew Morton <akpm@linux-foundation.org> > Signed-off-by: David Rientjes <rientjes@google.com> Acked-by: Vlastimil Babka <vbabka@suse.cz> Thanks.
On Fri, 17 Jan 2020 14:11:48 -0800 (PST) David Rientjes <rientjes@google.com> wrote: > If thp defrag setting "defer" is used and a newline is *not* used when > writing to the sysfs file, this is interpreted as the "defer+madvise" > option. > > This is because we do prefix matching and if five characters are written > without a newline, the current code ends up comparing to the first five > bytes of the "defer+madvise" option and using that instead. > > Use the more appropriate sysfs_streq() that handles the trailing newline > for us. Since this doubles as a nice cleanup, do it in enabled_store() > as well. I can't really I really understand this prefix-matching thing that we're taking away. Documentation/admin-guide/mm/transhuge.rst doesn't appear to mention it. Could we please add a paragraph to the changelog to spell all this out. Bonus points for formally describing the behaviour which we're removing! Thanks.
On Sat, 18 Jan 2020, Andrew Morton wrote: > > If thp defrag setting "defer" is used and a newline is *not* used when > > writing to the sysfs file, this is interpreted as the "defer+madvise" > > option. > > > > This is because we do prefix matching and if five characters are written > > without a newline, the current code ends up comparing to the first five > > bytes of the "defer+madvise" option and using that instead. > > > > Use the more appropriate sysfs_streq() that handles the trailing newline > > for us. Since this doubles as a nice cleanup, do it in enabled_store() > > as well. > > I can't really I really understand this prefix-matching thing that > we're taking away. Documentation/admin-guide/mm/transhuge.rst doesn't > appear to mention it. Could we please add a paragraph to the changelog > to spell all this out. Bonus points for formally describing the > behaviour which we're removing! > The current implementation relies on prefix matching: the number of bytes compared is either the number of bytes written or the length of the option being compared. With a newline, "defer\n" does not match "defer+"madvise"; without a newline, however, "defer" is considered to match "defer+madvise" (prefix matching is only comparing the first five bytes). End result is that writing "defer" is broken unless it has an additional trailing character. This means that writing "madv" in the past would match and set "madvise". With strict checking, that no longer is the case but it is unlikely anybody is currently doing this.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 13cc93785006..1c61dea937bc 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -177,16 +177,13 @@ static ssize_t enabled_store(struct kobject *kobj, { ssize_t ret = count; - if (!memcmp("always", buf, - min(sizeof("always")-1, count))) { + if (sysfs_streq(buf, "always")) { clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("madvise", buf, - min(sizeof("madvise")-1, count))) { + } else if (sysfs_streq(buf, "madvise")) { clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("never", buf, - min(sizeof("never")-1, count))) { + } else if (sysfs_streq(buf, "never")) { clear_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags); } else @@ -250,32 +247,27 @@ static ssize_t defrag_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) { - if (!memcmp("always", buf, - min(sizeof("always")-1, count))) { + if (sysfs_streq(buf, "always")) { clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("defer+madvise", buf, - min(sizeof("defer+madvise")-1, count))) { + } else if (sysfs_streq(buf, "defer+madvise")) { clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("defer", buf, - min(sizeof("defer")-1, count))) { + } else if (sysfs_streq(buf, "defer")) { clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("madvise", buf, - min(sizeof("madvise")-1, count))) { + } else if (sysfs_streq(buf, "madvise")) { clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags); set_bit(TRANSPARENT_HUGEPAGE_DEFRAG_REQ_MADV_FLAG, &transparent_hugepage_flags); - } else if (!memcmp("never", buf, - min(sizeof("never")-1, count))) { + } else if (sysfs_streq(buf, "never")) { clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG, &transparent_hugepage_flags); clear_bit(TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG, &transparent_hugepage_flags);
If thp defrag setting "defer" is used and a newline is *not* used when writing to the sysfs file, this is interpreted as the "defer+madvise" option. This is because we do prefix matching and if five characters are written without a newline, the current code ends up comparing to the first five bytes of the "defer+madvise" option and using that instead. Use the more appropriate sysfs_streq() that handles the trailing newline for us. Since this doubles as a nice cleanup, do it in enabled_store() as well. Fixes: 21440d7eb904 ("mm, thp: add new defer+madvise defrag option") Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Mel Gorman <mgorman@techsingularity.net> Suggested-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: David Rientjes <rientjes@google.com> --- Latest 5.5-rc6 doesn't boot for me, something to be debugged separately, so this was tested on 5.4. No changes in this area, however, between the two kernels. mm/huge_memory.c | 24 ++++++++---------------- 1 file changed, 8 insertions(+), 16 deletions(-)