Rob Norris c6be6ce175 abd_iter_page: don't use compound heads on Linux <4.5 ... Before 4.5 (specifically, torvalds/linux@ddc58f2), head and tail pages in a compound page were refcounted separately. This means that using the head page without taking a reference to it could see it cleaned up later before we're finished with it. Specifically, bio_add_page() would take a reference, and drop its reference after the bio completion callback returns. If the zio is executed immediately from the completion callback, this is usually ok, as any data is referenced through the tail page referenced by the ABD, and so becomes "live" that way. If there's a delay in zio execution (high load, error injection), then the head page can be freed, along with any dirty flags or other indicators that the underlying memory is used. Later, when the zio completes and that memory is accessed, its either unmapped and an unhandled fault takes down the entire system, or it is mapped and we end up messing around in someone else's memory. Both of these are very bad. The solution on these older kernels is to take a reference to the head page when we use it, and release it when we're done. There's not really a sensible way under our current structure to do this; the "best" would be to keep a list of head page references in the ABD, and release them when the ABD is freed. Since this additional overhead is totally unnecessary on 4.5+, where head and tail pages share refcounts, I've opted to simply not use the compound head in ABD page iteration there. This is theoretically less efficient (though cleaning up head page references would add overhead), but its safe, and we still get the other benefits of not mapping pages before adding them to a bio and not mis-splitting pages. There doesn't appear to be an obvious symbol name or config option we can match on to discover this behaviour in configure (and the mm/page APIs have changed a lot since then anyway), so I've gone with a simple version check. Reviewed-by: Alexander Motin <mav@FreeBSD.org> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Rob Norris <rob.norris@klarasystems.com> Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Closes #15533 Closes #15588		2024-03-25 16:51:54 -07:00
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freebsd	abd: add page iterator	2024-03-25 16:50:35 -07:00
linux	abd_iter_page: don't use compound heads on Linux <4.5	2024-03-25 16:51:54 -07:00