On Tue, 09 Sep 2014 18:53:55 +0530, Manavendra Nath Manav said: > Why is it so? Why can't kernel mode code handle the page fault and reload > the page from swap? Also, can page fault occur when kernel is executing in > process context and/or interrupt context? There's no inherent chiseled-in-stone rule that says "the operating systems kernel may not page fault", and in fact many operating systems allow it. The IBM OS/360 family, starting with VS/1 and MVS (as OS/360's MFT and MVT variants ran on hardware that didn't do virtual memory) clear through Z/OS 40 years later now all supported having part of their kernel be pageable. I've worked with several Unix variants that allowed parts of the kernel to be pageable. But that's a design decision that adds little real benefit, especially on today's large RAM systems - even a Raspberry Pi has enough memory that you don't really need to worry about making the kernel pageable. Cautionary tale: I once had a UTX/32 system that had routines for recovery from disk errors (in particular, recovering and forwarding of bad blocks to spare blocks was done by the host, *not* the device), and supported having about 1/3 of the kernel code be pageable (this was in 1985 or so, and a Powernode/9080 with 16M of RAM was a *big* system, so being able to put 500K of a 1.5M kernel out on disk was a big win for performance). I'll let you think about what sort of afternoon I had the day that we kept hitting an I/O error on a bad block in the swap area (which quite reasonably paused all I/O to the failing disk until the error recovery routine ran), while the block-forwarder module was swapped out.... (And I've had to debug similar dork-ups in VS/1, VM/SP, and MUSIC as well. Actually... hmm, yep. I think I've seen every single OS I've worked with in 3 decades that supported paged kernel end up shooting itself in the foot because the wrong thing was paged out at the wrong time. That stuff is *hard* to get right...) That sort of thing is why Linus decided Just Say No. ;)
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