Date: Tue, 8 Oct 2002 10:45:46 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: David Schultz <dschultz@uclink.Berkeley.EDU> Cc: Peter Wemm <peter@wemm.org>, Sean Kelly <smkelly@zombie.org>, hackers@FreeBSD.ORG Subject: Re: swapoff? Message-ID: <200210081745.g98Hjkam078883@apollo.backplane.com> References: <20020713071911.GA1558@HAL9000.wox.org> <20020713073404.9869A3811@overcee.wemm.org> <20020713115746.GA2162@HAL9000.wox.org> <200207131636.g6DGaoqh081285@apollo.backplane.com> <20021007153845.GA371@HAL9000.homeunix.com> <200210072347.g97Nl3Zo049415@apollo.backplane.com> <20021008113614.GA319@HAL9000.homeunix.com>
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:...
:detect when there is insufficient space. (I actually thought it
:was right the first time.) Can you see anything obviously wrong
:with my math?
:
:The code works fine in all of my tests, except that calling
:swapoff() when the system is under heavy paging load and has
:multiple swap devices sometimes leads to a few pages being missed
:by the scan. I think the problem is that some process allocates
:some swap and starts paging out just before the device is marked
:as off-limits. Am I missing a simple solution to this problem?
:(For now, I kludge around the issue by rescanning if there are
:still blocks remaining.)
Ok, I think the problem is in swap_pager_swapoff() and
swp_pager_force_pagein(). Another process may be manipulating
the swblock (or a prior swblock) while swp_pager_force_pagein()
is blocked.
In fact, the swap block can be ripped out from under
swap_pager_swapoff() if swp_pager_force_pagein() blocks. i.e.
the 'swap' structure may be invalid after you call
swp_pager_force_pagein().
This is a sticky situation because both the VM object and the
swblocks may be manipulated by other processes when you block. I
think what you need to try to do is this (it's a mess, if you can think
of a better solution definitely go another route!)
while ((swap = *pswap) != NULL) {
if (anything_is_swapped_to_the_device) {
try_to_page_it_all_in
(note that the swblock structure is invalid the moment you
block, so swp_pager_force_pagein() should be given
the whole range).
/* fall through to retry */
} else if (the_related_object_pip_count_is_not_zero) {
vm_object_pip_sleep(...)
/* fall through to retry */
} else if (swap->swb_count <= 0) {
free the swap block
*pswap = swap->swb_hnext;
}
}
-Matt
Matthew Dillon
<dillon@backplane.com>
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