After this documentation was released in July 2003, I was approached by Prentice Hall and asked to write a book on the Linux VM under the Bruce Peren's Open Book Series.

The book is available and called simply "Understanding The Linux Virtual Memory Manager". There is a lot of additional material in the book that is not available here, including details on later 2.4 kernels, introductions to 2.6, a whole new chapter on the shared memory filesystem, coverage of TLB management, a lot more code commentary, countless other additions and clarifications and a CD with lots of cool stuff on it. This material (although now dated and lacking in comparison to the book) will remain available although I obviously encourge you to buy the book from your favourite book store :-) . As the book is under the Bruce Perens Open Book Series, it will be available 90 days after appearing on the book shelves which means it is not available right now. When it is available, it will be downloadable from http://www.phptr.com/perens so check there for more information.

To be fully clear, this webpage is not the actual book.

10.5 Emergency Pools

Two emergency pools of buffer_heads and pages are maintained for the express use by bounce buffers. If memory is too tight for allocations, failing to complete IO requests is going to compound the situation as buffers from high memory cannot be freed until low memory is available. This leads to processes halting, thus preventing the possibility of them freeing up their own memory.

The pools are initialised by init_emergency_pool() to contain POOL_SIZE^10.5 entries each. The pages are linked via the page$\rightarrow$list field on a list headed by emergency_pages. Figure 10.5 illustrates how pages are stored on emergency pools and acquired when necessary.

buffer_heads are very similar as they linked via the buffer_head$\rightarrow$inode_buffers on a list headed by emergency_bhs. The number of entries left on the pages and buffer lists are recorded by two counters nr_emergency_pages and nr_emergency_bhs respectively and the two lists are protected by the emergency_lock spinlock.

Figure 10.5: Acquiring Pages from Emergency Pools

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Footnotes

...POOL_SIZE^10.5

Currently defined as 32.