Re: [Crash-utility] [PATCH 0/2] crash: warnings of excluded page structures

On Wed, Jan 08, 2014 at 03:32:01PM -0500, Dave Anderson wrote: > > > ----- Original Message ----- > > > > > > > > I have proposed a patch to makedumpfile to (optionally) exclude from > > a dump the page structures representing excluded pages. > > The idea being that a system with terabytes of system memory has > > millions of pages of page structures. And most of them are unneeded. > > > > That patch thread begins here: > > http://marc.info/?l=kexec&m=138853299130125&w=2 > > > > Dave [Anderson] raised these crash-related issues; > > Although I'm sure you tested this, I find it amazing that > > only the "kmem -[fF]" option is the only command option > > that is affected? > > If I'm not mistaken, this would be the first time that legitimate > > kernel data would be excluded from the dump, and the user would > > have no obvious way of knowing that it had been done, correct? > > If it were encoded in the in the header somewhere, at least a > > warning message could be printed during crash initialization. > > ... > > Right, but look at all of the other page struct offsets in addition to > > page.lru that are used. The page.flags usage comes to mind, and for > > example, what would "kmem -p" display for the missing pages? > > Or "kmem <address>"? And would "kmem -i" display invalid data? > > I'm just speculating off the top of my head, but the page structure is > > such a fundamental data structure with several of its fields being > > used, > > just enter "help -o page_" to see all of its potential member usages. > > > > So I am submitting two patches for your consideration, should the patch > > to exclude unused vmemmap pages be taken into makedumpfile. > > > > - [PATCH 1/2] crash: initial note of excluded page structures > > This one makes crash startup look like this: > > This program has absolutely no warranty. Enter "help warranty" for > > details. > > > > NOTE: Unused vmemmap page structures are excluded from this dump. > > GNU gdb (GDB) 7.6 > > Copyright (C) 2013 Free Software Foundation, Inc. > > > > - [PATCH 1/2] crash: kmem warnings for excluded page structures > > This patch modifies kmem options -f, -F, -s addr, -S addr, and -i. > > Those are the only options that I could detect looking for excluded > > pages. > > > > This patch applies on top of the first, and adds some warnings to the > > output of these kmem options. For example: > > > > crash> kmem -f > > Note: kmem -f may fail because unused page structures are excluded > > from > > this dump. > > NODE > > 0 > > ZONE NAME SIZE FREE MEM_MAP START_PADDR > > START_MAPNR > > 0 DMA 4095 3934 ffffea0000000038 1000 0 > > AREA SIZE FREE_AREA_STRUCT BLOCKS PAGES > > 0 4k ffff880000013068 2 2 > > ... > > > > crash> kmem -i > > PAGES TOTAL PERCENTAGE > > TOTAL MEM 128008147 488.3 GB ---- > > FREE 127599276 486.8 GB 99% of TOTAL MEM > > USED 408871 1.6 GB 0% of TOTAL MEM > > SHARED 11049 43.2 MB 0% of TOTAL MEM > > BUFFERS 5722 22.4 MB 0% of TOTAL MEM > > CACHED 44638 174.4 MB 0% of TOTAL MEM > > SLAB 62139 242.7 MB 0% of TOTAL MEM > > > > TOTAL SWAP 4893032 18.7 GB ---- > > SWAP USED 0 0 0% of TOTAL SWAP > > SWAP FREE 4893032 18.7 GB 100% of TOTAL SWAP > > > > Note: 1970727 free pages not found (excluded); results are incomplete. > > Unused page structures are excluded from this dump. > > > > -Cliff Wickman > > cpw(a)sgi.com > > Cliff, > > Can you make this patch far simpler? I would prefer that an > error message *follows* the gdb banner, i.e., where a number of > current warnings get displayed? You seem to be showing it just > above the gdb banner, where it kind of gets lost. Okay I'll move the message. > Secondly, I'm not sure where/how you are determining the 1970727 > pages that are excluded. Is it possible to put that in the > early warning message? The 1970727 pages were counted by the second patch. That is, I put a counter in the readmem path that counted 'excluded' errors. So there were 1970727 such errors during the execution of the kmem -i. It is probably not worth it to make a place for a new number in the dump header. Knowing how many million pages were excluded won't help if 'your' problem page was one of them. > > Thirdly, I'm not convinced that the kmem locations where you > are adding per-option warning messages are going to be the only > place where problems may arise. For that reason, I would > not even bother putting them in all those various locations, > but rather listing the "known" commands that may fail in the > early warning message. So do something like: Drop the 2nd patch then. If you think it might be more misleading then helpful I'm fine with just the initialization warning.